Update my side:
The Portable Power Station (Ecoflow River Plus, 360wh) arrived and I set it up. It is now powering my Raspberry Pi and the heat mat.
When only the Raspberry is running it draws about 3 watts according to the PPS, when both is running about 16w (the raspberry is controlling the heat mat to maintain optimal ground temperature).
I unplugged the PPS yesterday and had it run for the last 19h only on battery power. That discharged the PPS from 95% to 12%, so about 4.4% per hour. So it seems like a full power station could run my setup for about 1 day in case of an outage.
One caveat: I also have a IR lamp running that heats the area from above. In a real power outage this lamp would turn off and thus the vivarium would likely cool down a bit quicker than in this test. I didn't want to expose my snake to no overhead heat just to test the setup.

My takeaways:
  • The PPS solution does work as a UPS solution for the vivarium (beware that most PPS don't have a UPS feature like the Ecoflow)
  • The time that the PPS estimates is not accurate: the estimate when fully charged and running both raspberry and heat mat is almost 24h, but in reality, even with the heat mat switching on and off, the PPS barely provides 24h


Overall, I think this is a good way to give yourself peace of mind if you have a small snake collection and a generator / house-wide power solution isn't feasible.
Feel free to post questions and I'll try to respond.