How to leave a family sharing group --> https://support.apple.com/HT201081#leavegroup
If you downgrade, make sure you read the section "What happens when you downgrade or cancel iCloud+" in this support article https://support.apple.com/108318
What happens when you decrease your storage plan
● If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information, and iCloud backups won't complete. iCloud starts syncing again and iCloud backups resume when you increase your storage capacity or delete information to make more space.
As far as iCloud data are concerned from interpreting the wording of the above, in theory nothing will happen to your data except you won't be able to update it, etc. until you get a new plan. I have not seen anybody report anything to the contrary, though you will ultimately need/want to get upgraded storage of your own unless you downsize to the 5GB you get for free.
I don't have a TV subscription. I know that with Apple Music if you leave a family group and do not already have your own subscription then you will pretty much instantly lose any playlists you have set up. The solution in that case is for individuals to take out personal bridging subscriptions. If they later join a family that has a family level subscription they can then cancel their personal one if it isn't done for them automatically.
As you can see, some of your strategy depends upon what the "we" plan on doing down the line.
https://support.apple.com/108783 "If a family member is already paying for their own iCloud+ plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time."
(Sorry about all the bare URLs. Apple's forum software is supposed to automatically title them but that part has not been working the past week.)