While riding a mount summoned by Find Steed, when does the caster choose whether the mount is controlled or independent?

Question

Other Q&As on RPG.SE have discussed the by-now-familiar rules for mounted combat, including the differences between controlled and independent mounts. See, e.g., When can you choose to control a mount? Mounts summoned via find steed, however, are a special case. Per this 2018 guidance from Jeremy Crawford,

when you ride the mount [summoned via find steed] in combat, you decide whether it follows the rules for a controlled or an independent mount.

What remains unclear, however, is precisely when a caster who is already mounted on the summoned steed may, or must, make that choice. For example, it could be:

• When the DM calls for initiative rolls.
• Only on the caster's first turn.
• Only the mount's first turn.
• On any turn the caster takes.
• On any turn the mount takes.
• On any turn whatsoever.

Some of these options would require the caster to choose only once and stick with it. Others would conceivably let the caster change her mind each round, or even multiple times each round. Which (if any) is the correct way to handle mount control using find steed?

Any reasoned and supported answer is acceptable. If, e.g., Crawford's guidance is the only applicable source, then an answer well-reasoned from that -- especially one with experiential support -- is acceptable.

Answer 1

When you mount it

The initiative of a controlled mount changes to match yours when you mount it.

The choice of if it will be controlled or independent happens when you mount it. To change that you need to dismount and remount which takes your entire movement (half to dismount, half to remount) so it's not much of an impediment because, while mounted, you are unlikely to be using your own movement for anything anyway.

Answer 2

The description of the find steed spell says:

Your steed serves you as a mount, both in combat and out, and you have an instinctive bond with it that allows you to fight as a seamless unit. [...] While your steed is within 1 mile of you, you can communicate with it telepathically.

Anytime you are within that mile seems like a legitimate time. Discuss with your DM, but the Jeremy Crawford tweet you quote starts with:

when you ride the mount [...]

...so it sounds like when you get on the mount is the official option, but any of the 6 options you listed that fit into the narrative and ongoing action is always a great way to do it.