But L2ARC only helps read speed. The idea with dm-writecache is to improve write speed.
I started thinking about this when considering using a SAN for the disks, so that write speed was limited by the 10GbE network I had. A local NVMe could then absorb write bursts, maintaining performance.
That said, it's not something I'd want to use in production that's for sure.
There was some work being done on writeback caching for ZFS[1], sadly it seems to have remained closed-source.
That's what SLOG is for if the writes are synchronous or if you have many small files/ want to optimize the metadata speed look at the metadata special device, which can also store small files of configurable size.
ZFS of course has its limits too. But in my experience I feel much more confident (re)configuring it. You can tune the real world performance well enough especially if you can utilize some of the advanced features of ZFS like snapshots/ bookmarks + zfs-send/recv for backups. Because with LVM/ XFS you can certainly hack something together which will work pretty reliably too but with ZFS it's all integrated and well tested (because it is a common use case).
Worked well enough on the small scale, but sadly haven't had the time or hardware to test it in a more production-like environment.
Also, it was starting to feel a bit like a Jenga tower, increasing the chances of bugs and other weird issues to strike.