KeepassDX is great, can use it with just about anything too. I used it over sftp for a bit. It’ll happily do Google Drive, OneDrive, DropBox and just about anything that implements the right content providers.
Going through the provider is nice, it gives NextCloud an opportunity to sync it before it hands it over to KeepassXC, and knows when it gets saved too so it can sync it immediately. I don’t think I’ve had merge conflicts since, and I still have my offline copy just in case.
The annoying part is when you’ve added a password on one side and cleaned up a bunch of passwords on the other side. When they get merged, it doesn’t merge what changed it merges the databases together so your cleanup is gone. It’s safe at least, and exceedingly rare.