Only downside to using your own sync server for your devices' Firefox Sync is that the Firefox/iOS cannot accept custom Sync server URL; with Firefox/iOS, you are stuck with Mozilla Sync server.
That sounds equally like a downside to using Apple devices as a lot of open source self-hosted applications are limited in their respective iOS version.
I strongly suggest to read the requirements fully before commenting what amounts to misinformation at best.
You can run everything locally. You don't need spanner and can use mysql instead as a database. Also, as the previous commenter already told you, you can can run sentry locally.
Let's see whether you're willing and able to walk back your comments. I'm in the same boat as you - I'd want to run everything locally and I'm very happy to see it's entirely possible.