Figma is great, they have very interesting ways of solving problems, through rethinking from first principles of what makes a feature interesting or useful for the end user.

This looks fantastic, they are properly innovating and not just trying to be a web version of sketch. I just looked into how paths work (pen tool), and it is finally exactly how I originally envisioned they would work when I was first introduced to them decades ago. I never fully understood why the pen tool had a surprisingly high learning curve and general awkwardness. I assumed it was to meet some specific high-end needs of pro designers (I am an engineer first). Figma seems to be showing otherwise.

How about https://canva.com/ too.

Seems very polished to me -- but I'm not a web developer or graphic artist.

Does anyone know if there is an open source alternative to Sketch/Figma?

Not open-source afaik, bit closest one with Linux support that I found: Gravit Designer - https://designer.io/

If you want to design UIs (several pages, a single page is fine) Inkscape will prove painful.

As mentioned above, I believe the closest is Inkscape.