I just spent the last few weeks figuring out how to solve this problem for myself after running out of space on my free Google plan, so here goes.. I do have a NAS at home, so I was basically trying to move my backup from Google Cloud to my local NAS. Another consideration was that my wife should be able to use the solution so it needed to be simple and hands-off.
Probably the most difficult to solve for is Google Photos. Google offers 15 GB, and this gets filled up really fast if you take a lot of photos and videos. I don't, but still with every phone upgrade I've tried to move all files over to my new device, which results in space running out quite quickly. On top of that, Photos is actually _really_ nice. It works seamlessly, and offers features like facial recognition out of the box.
Anyway, I tried 3 different options - Nextcloud, Syncthing, and Immich.
Nextcloud was a little too complicated to setup, even with docker. Wasn't impressed by it.
Syncthing had an issue with Android app support. I think there are a couple of forks, but I didn't want to rely on that.
Immich was what I finally settled on. It is photos/videos specific, so doesn't work as a general purpose backup but I was more than happy just solving for this. My Google storage was being used roughly half and half by Email and Photos. Solving for half was good enough! On the plus side, Immich is a really impressive Google Photos replacement, and behaves almost exactly the same as the Google app, and has advanced features like facial recognition, dupe detection etc.
For email, contacts, SMS etc, I'm still relying on native Google backup.
Finally, there was Whatsapp. I had disabled Google backup a while ago, but wanted to preserve the photos from it. For that, I configured Immich to sync the whatsapp images folder with my library, and that's been working perfectly well.