I've done some digging and found several reddit threads about exactly the same battery drain problem. Some people were suggesting a hard reset, I've done that before but that made no difference. And interestingly, someone mentioned that it could be related to the Weather app complication syncing issues (that have been a proper pain in 26.0.2), suggesting that the constant unsuccessful sync attempts are causing the drain.
So I've done a proper cleanup. I've removed weather complications from all my watch faces, deleted multiple animated faces that I don't really use, then did another hard reset to kill any potentially remaining processes, and - knock on wood - it seems much better now! I don't wanna speak too soon - charged to 100% around lunchtime and now (~7pm) it's at 91%, that feels about right. Haven't had a chance to try a run today (sniffles... 🤧🙄) but with a bit of luck it's gonna be ok. Gosh, this watch was able to take me through ~50k in one charge, now it wanted to die after 15, that's not cool at all! - Anyway, rant over, I'll just leave this here in case anyone else stumbles upon the same problem.