I would not try to plan a way to replace the whole Apple ecosystem in one fell swoop. It’s just too much. Apple has really good integration between all their products, so you need to unwind it in as low friction a way as you can.
Apple Watch? That’s not going anywhere. Plan around it.
iPhone and iPad? They’re just hardware and they work. Don’t waste time trying to replace them. Changing a user’s OS is a great way to ruin everyone’s day.
iCloud? Here you can get to work. Immich can kill Photos (and save a ton on monthly cloud costs). Email? I would probably leave it alone, unless you just want to sidestep to another provider like Proton. iMessage? Good luck. Apple has that pretty locked down, and no long term user wants to try to get all their friends to install Signal or anything like that.
Try tackling photos first and go from there. You might discover that they pain of switching is so high it’s just not worth going any farther. Or you’ll learn how to navigate the bumps and can be better prepared for the next product you try to de-Apple.