Sounds like fun. Which tech stack? Depends.

• Wanna build a proof of concept quickly? Pick a stack you're familiar with.

• Wanna make a hobby project for fun? Pick whichever stack you want to learn better.

• Speaking of learning, want to improve your programming experience with the aspiration to be hired as a software engineer? Pick a popular tech stack.

• Want to build something that you hope will grow and will eventually look for contributors (whether that be hiring people or open sourcing)? Pick a popular tech stack.

There are other less important factors like technical pros/cons of the tech stack depending on what your application is doing and how you expect it to grow technically and as a product, but I would start with the above.
A lot of tech stacks out there can build a relatively successful Reddit-like app, and with enough experience, scale it when needed.
If you are less experienced and want to try avoiding some pitfalls and antipatterns that could make maintaining or scaling your app difficult, I would recommend picking a stack that contains a more full fledged framework that provides a lot of the code structure and tools for you.

503 means Service Unavailable, 500 is a slightly more generic Server error.
They're both likely to be intermittent when the server is under very heavy load, like it is lately with the reddit migration stuff going on.

Wait 5-10 minutes and refresh to try again. Try not to spam refresh every 5 seconds. If everyone did that, it would make the issue worse and more long-lasting.

Over time I'm sure we'll see server improvements :D