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.
Have you checked out the ODROID? https://www.hardkernel.com/shop/odroid-h3-plusAssuming you've got a spare SSD and RAM lying around, this could be a great option and is x86 rather than arm.
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/
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.