I've not looked exactly what OMV does, but years ago (when non-rotational media first became available) there were a number of settings for netbooks (with non-rotational media) to limit writes by increasing dirty write timeouts, decreasing "swapiness", and a few other configs, so perhaps this is what OMV has. But you still face writes it's just that they are more consolidated. So, by default you used to have writes every 30 seconds (unless you had a lot of data), and the increase was to 120 seconds. But that meant that you could lose a few minutes of data on failure since it's in core instead of flushed to disk. So then people did things like downgrade from ext4 to ext2 to avoid disk journaling, which then means you're back in the 90s style full disk check.
On my machine, I have a raid (actually a zfs pool so it has crc for fixing problems) for root between an ssd and a normal disk. The normal disk never spins down since I get occasional writes from cron, ntp drift, etc.
TVHeadend mostly runs in RAM but would write to the disk from time-to-time such as if you have an EPG update that matches an autorec then it would create a new file on disk.
You can test it out yourself by doing something like:
touch /tmp/mytimestamp
sleep 3600
find .hts -newer /tmp/mytimestamp -ls
So you create a timestamp file, sleep an hour and then find files in the .hts directory that are newer than your timestamp file (assuming you were in the tvheadend directory, wherever that is on your system).
You "sleep 86400" if you want to sleep for a day and then find all files modified.
My preference (depending on cash) would be buy a cheap ssd and put it in a usb enclosure. The reason is that you can then save live tv for rewind on that disk. It would also be silent and use virtually no watts.
Second preference would be to leave it on SD and just do a backup occasionally on to your usb disk. It just depends how much modifications you do. Perhaps you intend to do nothing outside tvh, in which case it is ok, but if you end up twiddling with different system settings then it makes it difficult to backup.
Third preference would be buy a second SD card + card reader and mirror RAID across the two so that when one fails, you can still read off the other. I don't think you can use zfs with only 2GB RAM and I've had numerous issues with btrfs causing me hours of grief that I couldn't recommend it, so basic md/RAID would be the best you could do.
Fourth preference would be install on the usb disk. The only reason this is my least favourite is because I think the disk would spin-up and down, causing failure, or just generally wear out.
Oh, or 2.5th preference, buy a WD RE4/RED (NAS drive) and use that inside an a usb enclosure since I'd be fairly confident that disk could survive 24x7 usage even inside a usb enclosure rather than inside a PC. But that would be the most costly, but they have a much higher MTBF (though Google concluded a few years ago that the MTBF of disks is nonsense) and also have a far higher "non-recoverable read errors" number (used to be 1*10^16 vs 1*10^14).