SyncThing - to sync my files between devices and avoid the big corporations cloud (use it for notes, Keepass database, photos, etc)

Amen, sibling.

Logseq - super advanced note taking with tagging and relationships between notes (all store locally)

Quite nice, although I'm still struggling to integrate it with my workflow. I just haven't taken the time to learn how to best use it.

Authpass - opening my Keepass password database on my phone

Do you trust it? Why?

I'm really cautious (nervous) about the program(s) I give access to my kbdx. On the desktop, I've actually code audited the tool I use; I can't as easily do that on Android.

GrapheneOS Camera - just to avoid the Google camera app as I have Google Photos disabled on my phone and needrd a good app that doesn't break when going to view the gallery (since it tries to open Google Photos)

I've been using OpenCamera for this, but recently started using PhotonCamera - it has a great UI.

Simple Gallery - to view my photos on my phone

That's a good one. I ended up with Aves because it allows fairly complex filtering, and the UI is nice.

Signal - I have it but I have very few friends that use it unfortunately

A few years ago, I talked my wider family into using Wire, and now we're all stuck on it. It keeps getting progressively worse with each release, and sooner or later I'm going to have to pick something to replace it. I don't know what that will be, but it will be federated, and it won't mandate IDs tied to a phone number.

Vinyl Music Player - to play my local mp3 files and playlists (I use MusicBee to manage what I sync to the phone as my mp3 library is quite large, and SyncThing to actually copy stuff over)

God, I seem to change music players every couple of months. I guess I've been using Metro for a while and haven't found the thing that irritates me and sends me on a search for a new player, so maybe it'll stick. That's interesting about MusicBee, though - I've been looking for something like that, so thank you!

I'll add to your list:

• PhotoBackup, which is a reliable and fantastic replacement for Photos syncing. This is then tied into PhotoPrism on the server, for web access and sharing. I had to cobble it together, mostly b/c of limitations in PhotoPrism, but it does replace the whole Goog Photos workflow.
• NewPipe, which is superior to the YT app in all ways, and is one of the best OSS all-around mobile apps.
• QKSMS, which I keep coming back to. I haven't found anything better yet, in any case.
• DAVx⁵. Unseen, but does some heavy lifting. One of the apps I make sure to support; replaces 1/3 of the critical "core" Google Android services
• Wireguard, which Just Works, and is always on.
• M.A.L.P., for when I'm playing music on the house system
• openScale, a quite nice OSS weight tracker that supports a lot of off-brand smart scales, without phoning home
• KDE Connect, what for sending stuff to/from other computers amd phones. Works great, even without KDE (mconnect is an alternative desktop service)
• Etar, as my calendar app. Been using it so long, I've forgotten it's not stock.
• ntfy replaces so much notification on my phone, and it does so reliably and well. Worth a shout out.
• SimpleTask, the best to todo.txt Android app. Another app that I've been using so long it seems almost stock.
• I've been using FlorisBoard for several months now. It has no text prediction, but has a ton of features I use. I try other keyboards every once in a while, but end up back on Floris. The only thing I miss is prediction, and with it better (any?) correction. Still, aside from DAVx⁵, it's probably the most used app on my phone, so...
• Geometric Weather! Such a great weather app.
• Material Files replaces the stock file manager. Great program, and while remote FSes are limited to ftp, it does the job.
• Wormhole William is a good between-devices file transfer tool.
• Termux, of course. Are there any real competitors?

Thing is, I'm running OEM Android, b/c I have a flip phone and don't want to lose the outside screen support by flashing. So I uninstall or disable nearly every built in app, and replace them with F-Droid apps (via Droid-ify). It's not much, but it's an honest living.

Edit MusicBee... oh, Windows. Oh, well. The search continues.