I use the FUTO keyboard made by the same people who made grayjay and it in my experience works better than either open board or heliboard

I am on a Samsung s21+ and managed to push it pretty far

I would suggest downloading libre-Find, it isn't super good now for helping find Foss alternatives for apps but it keeps getting better

On its tracker I am 98% foss but I have it set to exclude the few offline only cracked/pirated apps I have on my device

Get a NewPipe fork, like Bravepipe or PipePipe. They include sponsor block built it which will enhance the viewing experience.

I would suggest using Heliboard as it includes suggestions and more options in settings.

Lastly, albeit a bit more complex, if you use a VPN, then you can install a SearxNG instance on your phone with Termux. Have your own search engine straight out of your phone. I would not suggest to do this without a VPN since you should switch your location often enough as to not be profiled. You can likely find tutorials online on how to do this. Or you can wing it if you are a Linux power user and know your way around GitHub and Terminal.

I had issues with FOSS calendar. It was so bad I had to use e/OS native one which is not great. Can't remember why, but whatever it was made me miss an appointment

Definitely second switching to heliboard- open board is abandoned unless it got picked up by someone, heliboard is a fork thats actively maintained and has made a bunch of improvements

Its a strict upgrade, I think you'll be happy with it :)

If you dont have a gallery app I really like both aves libre and lavender photos. Lavender is newer but the Dev has been super nice every time I've made a bug report or something. Aves is super feature complete and works really well. Both have a great user interface and user experience