I’m pretty sure I have read that after you transition your Apple ID to an iCloud login that you are stuck with that.
I have been considering moving as well because Gmail doesn’t work well with Apples mail app, but I’m not sure I want my Apple ID permanently stuck to iCloud.
Make sure you stay on top of keeping your card updated, I was on the intro price for family plan for years but got a replacement card and forgot to update Google and had to re subscribe at the regular rate.