I do software development both for myself and as an independent contractor as my FIRE career. One thing that I think is still ripe as an independent developer is applying specialized knowledge from a career to niche products. For example, my sister and her husband own an indoor play place so I built a custom tablet-based waivers, marketing, and loyalty system with my brother in law. It fits the exact needs a children's play place has much better than a generalized waiver product, it's a manageable project size, and it's a product category of one. We sold it as a service to the chain that they're part of as a whole, and may stop being lazy some day and sell it to other play places.
In another example, a welding engineer friend is trying to get me to work with them on a custom weld planning and inspection application. But I turned that down because I'm having too much fun on my embedded systems contracts right now.
So yeah, I think indie development of that kind of niche product is a great avenue to pursue.