it's pretty weak (about 1 hour with the server + router + Smartthings Hub plugged in) and I would have liked to keep the router and ST powered for as long as possible (security reasons).
but yes, apparently there is no other solution than to configure the NUT pluging to shutdown OMV when the battery is running low and not immediately after a power failure.
this ensures that if the outage is long enough, the UPS will shut itself down and so the OMV will restart on power resume.
the downside is that the router and ST Hub will not stay powered for as long as I'd like them to.
there is maybe a solution but it does not involve just the UPS and the server.
I could put a smart plug between the UPS and the server
on power failure, the Home Assistant running on OMV server will inform ST that it needs to turn off the plug after a delay
then the OMV will shutdown as instructed by NUT and then the smart plug will shutdown, cutting power to the server
I haven't figured out yet how to automatically turn on the smart plug when power returns; I guess I could turn it on manually, or maybe use a smart bulb that is always off as a trigger (smart bulbs power ON after a power failure)
so, pretty complicated but it could work :-))