As someone self taught in both (okay some schooling but I don't use much beyond some of the principles/design I learned), even learning frameworks you need to understand HTML, CSS and JavaScript. Okay maybe less JavaScript with something but I don't know them.

I picked Angular for something, maybe not the best, uses typescript which I like. But in the end it's just to hold variables. After awhile I could easily write something to dynamically create a form in the way I wanted. There are other things I use but if I really wanted I could probably do it all my own in JavaScript anyways.

That's why you learn them, without HTML you don't know not to put things on a screen, without css you can't organize or style it, without JavaScript you have no interaction. Learn them and you'll figure it out.

If you already an expert in python, you can keep doing that and build a frontend in python using Dash. They have bootstrap components and mantine (React) components for options of style you might prefer. Don't need to write JS or HTML. It's enterprise grade and widely used by large enterprise projects.

Another is Reflex but their open source version is tricky to run in production. They promote more of their cloud version or enterprise license.

Maybe try a full stack option like NextJS/NuxtJS, as a sideways step into frontend

I would first ask what you expect your frontend to do. Will your UI get live updates? What do you expect will be an average session time, minutes or hours?

I “just” want to build a pretty standard, responsive, modern-looking UI.

You can accomplish that with plain HTML, CSS and JavaScript. https://developer.mozilla.org/en-US/docs/Learn_web_development can be a good place to start.

Ideally without too much boilerplate

What is considered boilerplate, and too much of it, is a bit subjective.

in a framework that “feels good”

That is very much subjective. Also, you might not need a framework.

in a way that might at some point attract other contributors as well

Now you're considering your own tastes, as well as the tastes of an unknown future stranger :D I'd say don't worry about it.