What's a good Google Drive replacement for syncing my Keepass database?

Seconded. Great bit of software.

I have syncthinged my desktop (Mint as well) and my smartphone with a Raspberry Pi (DietPi!) as middledevice. If I change something on one of the two, the file gets synched to the Raspi, which then gives it to the other device as soon as it's online. This works great since a couple of years.

In the case of house burn down, because Syncthing stores a 1-1 copy of the folder on every device in the network, you would still have your data even a single device survives the incident, such as a phone, or a laptop

Second Syncthing, it is very fast, reliable, and flexible.

I used it coming from FileSync and Dropbox, and I had to change the way I thought about my shared folders to architect a good system for me. Eg: each root shared folder should serve a particular function that determines which devices it should be shared to (does this share need to be accessible in your phone? Laptop? PC? NAS?).

FYI you can set up untrusted peer sync to have your files all synching to another device (SFF device at your friend or relatives house, or a cloud server). That eliminates the concern of your house burning down, while keeping all of your Syncthing data secure and not worrying about it being stolen or accessed. If your house burns down you can connect back to the untrusted peer sync, put in your passphrase, and your data will all return.

https://t-shaped.nl/untrustedpeerencryption

Only downside to this is that if your house burns down you’ll lose everything - but a friend suggested me to have important files on an encrypted tarball stored in the cloud.

For those with lots of files and poor upload speeds but blessed with a desk at work, also consider stashing an encrypted disk in a drawer / fake plant / etc.

The amount of headaches I had setting this up... I can't tell you how hard I tried.

I think in the end I figured out it doesn't like vlans very much if you don't want to use their relay.

I used to use Filen for this, but it never worked very well. The file provider path it returned to Keepass2android was only temporary, so it would break periodically. Did Filen change how that works?

I eventually started using Syncthing instead. I connect to my home wi-fi often enough that it's never too far out of sync with my home PC. And since it's a local file, there's no issue with using absolute paths.