While crypto was and still is my turf, I think I could also do well in the stock market. The problem is that the entry barriers in the stock market are quite large. By my estimates, it will cost between 10k and 100k a month to run an HFT strategy fast enough to compete with the fastest players in the field (e.g. running market making strategies). If you have good alpha you could probably get away with slower and cheaper access.
In the crypto world, the market access is free for all, and everybody has equal standing (from what I know). After implementing some WebSocket/JSON APIs you get access to the market that turns around 100s of millions USD per day. Of course, it's much smaller than the stock market, but it's real nevertheless.
I think it's a myth that smaller strategies cannot compete with established HFT firms. It's also a myth that you need grad level knowledge of math/statistics. It might even hurt, becuase phds will be prone to "do things the right way" as opposed to "do things that work". I think it's also a myth that HFT firms hire exceptional talent. In fact, most firms have rather mediocre staff. The reason is that most firms don't make exceptional money. So they don't have a salary pool large enough to pay exceptional people exceptional wages. I'm talking upward from 300k. Companies like Google will happily pay skilled engineers around that watermark. The Google/startup jobs might feel dull, but the finance jobs are way more boring and less rewarding... I can rant on this forever - lol.
That being said, I consider myself mediocre developer as well. Couple months ago I applied for Senior Developer jobs at 3 firms and didn't get a single job offer. I didn't try hard, didn't prepare for the interviews, but still.
Edit: I applied for these jobs just to see what's up. Neve intended to take the jobs.