I went from software engineer to founder. As part of that journey I had to learn something about design since my co-founder didn't have that skill or interest. My advice for founders: copy. Find a design you like, brand colors you like, and just copy it exactly. Colors, shadows, spacing, layout. As long as it is an established brand/company, the UX will be good enough that it won't harm your startup. And that's all y…
If you can spend money on marketing, you cannot afford to not spend money on ensuring the UX is working for your users and to get insight into if poor design is the reason behind not performing as expected for your bottom line.
Pro-tip: UX is NOT UI (there are a lot of off the shelf design systems that make designing UI trivial e.g. Salesforce lightning design system, material design, etc. - hire someone who is a UX design generalist until you can afford specialists - UX designers/researchers).