You say there’s no good Firefox browser for Android, but I strongly recommend you consider Mull. It’s pretty much Firefox w/ Arkenfox but for mobile and it works exactly as it should. For Mull you should add the DivestOS repository to F-Droid because the F-Droid one is outdated.

I like Fossify’s apps for phone and contacts. As for photo gallery, I really like Aves Libre, paired with Image Toolbox and Open Video Editor for light edits.

I don’t use an RSS aggregator myself, but Feeder tends to be rated well.

You pretty much have to get into self-hosting to truly have great replacements.

Google music - tidal, or qobuz are the same thing but not as bad as Spotify. Otherwise you can self host navidrome or jellyfin->finamp

RSS Feed: choose from like 10 Foss projects on fdroid

Google home: Home assistant is self hosted and like 10x better and can integrate with more things

PDF scanner: Microsoft Lens (not FOSS) but really really really good, doesn’t require a Microsoft account, doesn’t have extra permissions, and barely has telemetry. If you are OK with >10x slower, there is OSS Document scanner

VPN: ProtonVPN, Mullvad, maybe PrivateVPN. You have to pay for one. The app is wireguard or openvpn

Google photos: Immich is pretty much a perfect replacement

WhatsApp you literally can’t get away from. Signal is the easiest replacement, but i could only get mg core friends on it.There is no way 99.9% of people can get their friends and family to switch to matrix or XMPP.

Proton also has a VPN

I use Read You for RSS

For WhatsApp and other platforms, I host a Matrix server and add bridges for them.

To replace Google Home, I host a Home Assistant server

For music, you can host your on music streaming service with Navidrome and use an app like Ultrasonic on your phone. If you don’t want to buy music outright, you can get a qobuz subscription and use qobuz-dl to grab music from it.