I've investigated this in some depth for my personal boxes with slow HDDs, in lieu of a more formal writeup, here is the best solution I've found. First:
echo "write through" > /sys/block/sda/queue/write_cache
This lies to the OS and tells it that drive writes don't need to be fsync'd. Replace "sda" with whatever your drive in question is, naturally. Note that this is not persistent, you'll need to configure your init system to do this on boot. You can verify it's working by looking at /sys/block/sda/stat (see
https://www.kernel.org/doc/html/latest/block/stat.html).
Next, in /etc/fstab configure your filesystem to be mounted with "barrier=0" (note that only some filesystems support this), which will often prevent data from getting written out to the disk at all, instead getting kept in cache.
You still need the first part because the filesystem layer won't cover all possible cases--for instance, LVM thin provisioning will issue a manual flush below the filesystem layer once per second, and there's no way to remove that.
One problem I haven't managed to solve is detection--in the unlikely event things don't shut down properly (e.g. a kernel panic), how do I find this out so that I can restore from a backup (rather than having something subtely corrupted somewhere)? This is conceptually easy with some global bit in a special sector used as a mutex, but I don't know of any existing off-the-shelf solution implementing this.