I'm not aware of any great FOSS/FLOSS Tasker alternatives. There are a few options, but they will be less capable, functional, extensible, user friendly, or modern.
More direct alternatives
Requires a server to run automations/dcripts
Requires scripts and may require a server and/or additional add-on apps
I believe you're looking for Easer, though not sure how well it matches to your criteria https://github.com/renyuneyun/Easer https://www.f-droid.org/en/packages/ryey.easer/
I'm curious - why do you hate Tasker? I've been using it for years (well before Joao became the dev) and I've always found it supremely useful.
The UX is so terrible, I can't figure out what's to like about it.
It's still a useful app, I mean it's design is complete garbage, but you get utility.
Ah, fair enough. I've been using it long enough that I guess the UX doesn't bother me any more. And the benefits from the ongoing development are still very good IMO.
After many years of using Tasker, I switched to Macrodroid.
Not quite as powerful but does 95% of what Tasker can do and the interface makes waaaay more sense, to me.
Macrodroid can also utilise Tasker and Locale plugins.
From what I can see, Macrodroid does not appear to be opensource, but thank you for the suggestion.