The filesystem should do files, if you want something more complex do it in userspace. We even have FUSE if you want to use the Filesystem API with your crazy network database thing.
The extended (not extant) family (including ext4) don't support copy-on-write. Using them as your primary FS after 2020 (or even 2010) is like using a non-journaling file system after 2010 (or even 2001)--it's a non-negotiable feature at this point. Btrfs has been stable for a decade, and if you don't like or trust it, there's always ZFS, which has been stable 20 years now. Apple now has AppFS, with CoW, on all their devices, while MSFT still treats ReFS as unstable, and Windows servers still rely heavily on NTFS.
As for ensuring data integrity, I cannot talk about other CoW filesystems, but ZFS has an atomic transaction commit that relies on CoW. In ZFS, your changes either happened or they did not happen. The entire file system is a giant merkle tree and every change requires that all nodes of the tree up to the root be rewritten. To minimize the penalty of CoW, these changes are aggregated into transaction groups that are then committed atomically. Thus, you simultaneously have both the old and new versions available, plus possible more than just 1 old version. ZFS will start recycling space after a couple transaction group commits, but often, you can go further back in its history if needed after some catastrophic event, although ZFS makes no solid guarantee of this (until you fiddle with module parameter settings to prevent reclaim from being so aggressive).
If it counts for anything, I have hundreds of commits in OpenZFS, so I am fairly familiar with how ZFS works internally.