You are probably fine for balance, but might not get what you want
Why this might not be the right homebrew for you
You say you want "...strong... Heavy armour melee/mid-field support" But you want to achieve this by swapping the classes spell lists.
Druids can't wear metal armor, don't have access to spells like shield or mage armor, this makes them rely on dex to have some semblance of defense. Low defense, low strength goes against your stated goal. To achieve your desired goal, you basically need to get rid of the metal armor restriction. But that is a major boost to the druid (+2 or +3 AC depending on dex).
Perhaps you can talk to your DM about giving up the traditional wildshape for lifting this restriction.
But at this point, we have basically taken the druid, and turned it into a cleric, just with a very strong subclass.
From what you describe, the cleric sounds like the right class for this character. So before going all in on the star druid. Consider if any of these could be a simpler fix for you:
- Is there a way you can change the twilight cleric into something you do like?
- Can you re-flavor another domain, that you do like the mechanics of, and just rename it.
- Cleric domains are relatively easy to homebrew, they all follow a set structure, can you make your own domain, possibly inspired by the star druid?
Would it be unbalanced
Now I am still going to attempt to answer your stated question, note that I am only considering swapping the spell lists, not any lifting the metal armor restriction or anything else.
As full casters, both druid and cleric tactics are largely dictated by their spell list. Swapping the two lists, I would expect the druid to play more like a cleric.
To discuss balance, we must have a balance point, we must ask our selves, is this weaker or stronger than x. I am going to use the cleric as the balance point, because I would expect this character to play more like a cleric.
Our druid gets a lot of utility out of wildshape, and the extra castings of guiding bolt, plus the archer form makes us a range focused character. We have low defense, so a traditional cleric would typically do less damage, but be more tanky. Wildshape however lets the druid be one of the best scouts in the game and probably puts this in the favor of the druid.
Level 5-9: Spirit Guardians is an amazing spell, that incentivizes both our homebrew druid and a regular cleric to get close to the melee, but the cleric having the better defenses, they will probably come out on top. At this point we will probably start using Dragon form, to make sure we keep concentration on spirit guardians, and use the spiritual weapon spell to attack with our bonus action. While the traditional cleric probably have to spend a feat on resilient constitution or warcaster to make sure they don't lose concertation. There isn't really any feat we can take to increase our defense, because of the no metal armor restriction, so best we can do is a dex boost for AC or wisdom boost for spell save DC.
Level 10+: We can now fly in our dragon form, making it a lot easier to stay safe while using spirit guardians, at least when there is a high ceiling or we are outside. At this point it is important to note that any full caster is very strong at level 10+, I think with the addition of a fly speed, we can mitigate the druids major weakness of low AC enough that we can give a slight edge to the druid, but not much.
Overall, I don't think it will be a major balance concern, but starting out with a range focus, and ending out hovering above the battlefield just out of melee range. Doesn't sound like the character you wanted to make.
As a final piece of advice, this a major change you are considering, talk with your group and your DM before the game starts. If one spell I haven't considered turns out to do something broken with druid features, just ban that one spell. Nothing wrong with banning a spell mid campaign as long as all parties agree the game will be better without it.
Agree from the beginning how to go about it, will you talk about it after the session, or will you ban mid session if something comes up.