This seems to mean adventurers who get within 5' of an enemy stay there until the enemy is dead, to avoid getting an opportunity attack.

• In a recent encounter, the tank threw the halfling at a slime (it was on the ceiling, out of reach...). They succeeded appropriate rolls, and after the combat I asked where they'd want to land. They placed themselves adjacent to the slime, as they'd be hit by an opportunity attack otherwise. I left it, as I wasn't sure I wanted to set a precedent, but I felt that they should probably land further away.
• A mounted character charged into combat. Despite having plenty of move left, they ended turn next to the enemy. It feels more RP that they should pass the enemy and end turn out of melee range?
• A large enemy tied down most of the players; even those which were badly wounded stayed in combat, aiming to defeat the enemy before their next turn, rather than pulling out and letting the other characters handle it. It feels unnatural that if a large monster is fighting ~4 players in melee, they'd be free to get an opportunity attack if one moves out of range – particularly if that character has moved to the back/side.

• Allowing a free disengage if a player enters combat 'with some momentum' (e.g. mounted or otherwise moving faster than normal). Probably I'd limit this to the initial charge into combat for the mounted character. (This When a controlled mount takes the Disengage action, does the rider still provoke an opportunity attack if he or she does not also Disengage? seems to suggest this would be OK for mounted characters?)
• Allowing a free disengage from behind a (large+?) creature if other players are also in combat with it and thus presumably distracting it.

Does this break things? Is there anything else I should be doing to make the combat more dynamic?

[EDIT: This understanding was slightly wrong – forced movement is excluded]

This seems to mean adventurers who get within 5' of an enemy stay there until the enemy is dead, to avoid getting an opportunity attack.

• In a recent encounter, the tank threw the halfling at a slime (it was on the ceiling, out of reach...). They succeeded appropriate rolls, and after the combat I asked where they'd want to land. They placed themselves adjacent to the slime, as they'd be hit by an opportunity attack otherwise. I left it, as I wasn't sure I wanted to set a precedent, but I felt that they should probably land further away. [NOTE – have now realised this is already exempt from AoOs] *
• A mounted character charged into combat. Despite having plenty of move left, they ended turn next to the enemy. It feels more RP that they should pass the enemy and end turn out of melee range? [NOTE – have now realised this is already exempt from AoOs] *
• A large enemy tied down most of the players; even those which were badly wounded stayed in combat, aiming to defeat the enemy before their next turn, rather than pulling out and letting the other characters handle it. It feels unnatural that if a large monster is fighting ~4 players in melee, they'd be free to get an opportunity attack if one moves out of range – particularly if that character has moved to the back/side.

• *Allowing a free disengage if a player enters combat 'with some momentum' (e.g. mounted or otherwise moving faster than normal). Probably I'd limit this to the initial charge into combat for the mounted character. [EDIT: I'm happy that most of these scenarios are covered by the existing force motion exclusion *
• Allowing a free disengage from behind a (large+?) creature if other players are also in combat with it and thus presumably distracting it.

Does this break things? Is there anything else I should be doing to make the combat more dynamic?

Updates

Noting some updates to my understanding, both for anyone still answering, and for the benefit of anyone who finds this question in the future:

@Cubic pointed out that forced movement doesn't risk an AoO; the rules specifically give 'gravity' as an example. Most of the 'cool' moves would be covered by this, including the thrown halfling.

It seems there's different ways of playing mounted characters; I'm going to change how I play them to allow the rider and mount's movement & actions to be interspersed, so the mounted character can do a 'rideby' attack without risking an AoO. (When a controlled mount takes the Disengage action, does the rider still provoke an opportunity attack if he or she does not also Disengage?)

My thoughts about disengaging from behind large creatures which are distracted by other PCs still remain. If a dragon has 3 PCs stabbing it in the front, should it get an AoO against one behind it disengaging? (I'm using the optional facing rules – particularly for large monsters, if the players put some effort into getting behind it)

I'm the DM, and I'm concerned that combat feels pretty static.

As I understand it in RAW:

• Once you've reached melee range, moving further away will provoke an opportunity attack, unless you disengage
• disengage costs an action (unless you have a feature that lets you do it for free)

This seems to mean adventurers who get within 5' of an enemy stay there until the enemy is dead, to avoid getting an opportunity attack.

This seems to make combat rather static, which I find irritating. My own experience of combat in HEMA suggests you frequently disengage beyond 5' even in 1:1 fights; with multiple opponents even more so. I feel it also limits options for players.

Some recent instances which annoyed me –

• In a recent encounter, the tank threw the halfling at a slime (it was on the ceiling, out of reach...). They succeeded appropriate rolls, and after the combat I asked where they'd want to land. They placed themselves adjacent to the slime, as they'd be hit by an opportunity attack otherwise. I left it, as I wasn't sure I wanted to set a precedent, but I felt that they should probably land further away.
• A mounted character charged into combat. Despite having plenty of move left, they ended turn next to the enemy. It feels more RP that they should pass the enemy and end turn out of melee range?
• A large enemy tied down most of the players; even those which were badly wounded stayed in combat, aiming to defeat the enemy before their next turn, rather than pulling out and letting the other characters handle it. It feels unnatural that if a large monster is fighting ~4 players in melee, they'd be free to get an opportunity attack if one moves out of range – particularly if that character has moved to the back/side.

I'm thinking of:

• Allowing a free disengage if a player enters combat 'with some momentum' (e.g. mounted or otherwise moving faster than normal). Probably I'd limit this to the initial charge into combat for the mounted character. (This When a controlled mount takes the Disengage action, does the rider still provoke an opportunity attack if he or she does not also Disengage? seems to suggest this would be OK for mounted characters?)
• Allowing a free disengage from behind a (large+?) creature if other players are also in combat with it and thus presumably distracting it.

Does this break things? Is there anything else I should be doing to make the combat more dynamic?

I'm the DM, and I'm concerned that combat feels pretty static.

As I understand it in RAW:

• Once you've reached melee range, moving further away will provoke an opportunity attack, unless you disengage
• disengage costs an action (unless you have a feature that lets you do it for free)

This seems to mean adventurers who get within 5' of an enemy stay there until the enemy is dead, to avoid getting an opportunity attack.

This seems to make combat rather static, which I find irritating. It seems static compared to my martial arts experience (HEMA), or to say video games. (I realise 5e is a different thing to these, but it leads me to wonder if combat is unintentionally static, given the disengage action has been provided).

Some recent instances which annoyed me –

• In a recent encounter, the tank threw the halfling at a slime (it was on the ceiling, out of reach...). They succeeded appropriate rolls, and after the combat I asked where they'd want to land. They placed themselves adjacent to the slime, as they'd be hit by an opportunity attack otherwise. I left it, as I wasn't sure I wanted to set a precedent, but I felt that they should probably land further away.
• A mounted character charged into combat. Despite having plenty of move left, they ended turn next to the enemy. It feels more RP that they should pass the enemy and end turn out of melee range?
• A large enemy tied down most of the players; even those which were badly wounded stayed in combat, aiming to defeat the enemy before their next turn, rather than pulling out and letting the other characters handle it. It feels unnatural that if a large monster is fighting ~4 players in melee, they'd be free to get an opportunity attack if one moves out of range – particularly if that character has moved to the back/side.

I'm thinking of:

• Allowing a free disengage if a player enters combat 'with some momentum' (e.g. mounted or otherwise moving faster than normal). Probably I'd limit this to the initial charge into combat for the mounted character. (This When a controlled mount takes the Disengage action, does the rider still provoke an opportunity attack if he or she does not also Disengage? seems to suggest this would be OK for mounted characters?)
• Allowing a free disengage from behind a (large+?) creature if other players are also in combat with it and thus presumably distracting it.

Does this break things? Is there anything else I should be doing to make the combat more dynamic?