@gnomekat
I don't know about Discord, but you could try https://programming.dev/c/math@lemmy.ml and see if it's what you're after
@gnomekat
It's always difficult to tell what "amateur" means when people apply it to maths because there's an enormous range of conception (and misconception) around what it means to 'do' maths, BUT you might find that the Finite Group (https://finitegroup.co.uk/) fits. There's a free tier which allows access to the Discord which is populated by people who can be pretty hardcore but also amateur, recreational, and other levels of engagement with maths. I've never found it to be anything other than welcoming: it's a bunch of people who enjoy maths and want to explore it with other people who enjoy maths.
If you're open to something in the real world, you could see if there's a MathsJam near you (https://www.mathsjam.com/find-a-jam/). If not, you could consider setting one up!