Anything other than QNAP.

Eaton is my recommendation. Look at the 5S1500 (modified sinewave) or 5SC1500. They all work with NUT. I have one of my UPS connected to a Raspberry PI Zero with NUT. And my servers NUT clients are scheduled to shutdown after a 2min outage and my NAS after 4min. I can also monitor the UPS from Home Assistant and receive notifications on my phone each time the UPS goes OB or OL.

https://www.eaton.com/content/dam/eaton/products/backup-power-ups-surge-it-power-distribution/backup-power-ups/eaton-5-series-ups-brochure-br153013en-lr.pdf

Any specific reason to move from a Mikrotik to OPNsense?

I think this is the best homelab router out there. If you are new to Mikrotik there is definitely a steep learning curve.

Openwrt is fairly good too, but I think documentation can be lacking and confusing for some edge applications. My other concerns with openwrt is performance since it is compatible with a wide variety of hardware is difficult to know how it will perform without testing it.

Can you share the link for that case?

In my experience Eaton UPSs are better than APC, CyberPower. Look at 5S1500LCD and the like.

I had no idea this was so common.

I have a project on mind and I was planning on using nomnoml directly. https://kroki.io/ seems to a better way to interact with it. Thank you for sharing.