How to do design as a technical solo founder

Steal designs! If you're confident with your coding skills, you should be able to replicate most of them and customize them to fit your use case. 😁

I use http://collectui.com/ to find UI designs.

If you're working on a web app, I would just use themes and templates. They are usually inexpensive ($20-$50) and look fine.

I'm also a developer / solo founder and I used to attempt my own design but eventually gave that up.

There is so much to do above and beyond design and development when you are starting a business that you will need to outsource anything that is not your core skillset.

Design is one of the easiest tasks to outsource, you can find many very talented designers on Upwork for very reasonable prices and you can browse Thier portfolios before hiring.

I think bootstrap gives you a lot of components with okay design. At the beginning, it is more about getting the functionality right and people want to use it.

I would say think first about the community you are trying to reach with your product. Design matters to designers. To others, they will want to see a website that looks like the sites they often visit. Taking Amazon UX as a reference is not bad. Ugly as hell but a known entity, aka builds trust with customers faster.
You can go pretty far with a default bootstrap theme. I would focus on the number of clicks it takes to reach a feature, what appears above the fold, etc. I also learned the hard way that many people get hang up on words, grammar and spelling so I would make sure the copy is easy to read before investing much resources in actual design - unless you are selling a product to designers of course :).

aesthetic design, or usability design?

If you strictly follow something simple like 'task centered system design,' you will build up some foundational ux design skills and everything you make will at least be usable. Transition to freestyle or goal oriented system design afterwards. Finally, add in some usability heuristic evaluations and you're in a good spot (there are 10 traditional ones, easy to find on duckduckgo).

Aesthetically speaking however, I dunno. I try to get my rude friends to complain about things until I've tweaked it enough that they grudgingly think its decent. Rude friends are indispensable.

Here's what I've been doing... I'm working on this:

https://getpolarized.io/

... take TONS of screenshots of designs you like. Save it to a folder called 'product design ideas' or something and use git or something and make sure it never goes away.

When you get stuck you can use this to scroll through for inspiration.

Check out dribbble tabs when you get stuck. Some of the designs there can inspire your designs of course.