I've been using Actual Budget in a docker container. It's worked well for me but not sure if it checks all the boxes for you.

here's a decent list of some public server instances you can look into. Tells you where they are located, if there are any rules or a privacy policy, what version of the server they're running, and some other stuff like that

https://joinmatrix.org/servers/

Aegis is a good one for Android. I use the totp field in my keepassdx database that I open with a password (or fingerprint) and my yubikey to store my auth codes. I use this with syncthing running on a raspberry pi so it syncs the password database across my phone and all my computers.

Edit: initially said keepassXC I meant keepassdx for the mobile app. Xc is the desktop version.

I bought one of the tall meerkats in 2021 and have had no issues with it so far. The thing is fantastic for a work from home setup. It barely takes up any space on my desk and runs so smoothly for anything from basic office tasks (libreoffice, only office, office365 pwa whatever ya use), to analyzing MRI datasets, to light gaming, i.e. Stardew valley type games. Absolutely love the thing!

Edit: would also like to add that pop is super user friendly and a great choice as a beginner distro

Have you checked out the ODROID? https://www.hardkernel.com/shop/odroid-h3-plus

Assuming you've got a spare SSD and RAM lying around, this could be a great option and is x86 rather than arm.