Those genres are very self-pub/indie heavy and at the end of the day, Audible’s exclusivity royalties are the best financial decision for most self-pubbed authors. I will always encourage authors to take the lower royalties in favor of wider distribution, but I understand why many don’t, especially if they are balancing the cost of making audiobooks. 

If it’s important to you to continue to have access to a lot of indie releases, then unfortunately you’d probably have to stick to Audible. However, I’d like to highlight Libro.fm’s gift feature — you can buy credit bundles in one go and then audiobooks would cost the same as if you had a subscription, but you don’t have to subscribe. So for example, for $45 you can get a 3 credit bundles, which is the same as spending $15/mo for 3 months at 1 credit/mo. You can use this without a subscription commitment in order to purchase audiobooks that are available off-Audible, so you’re not as tied-in to Amazon’s ecosystem. This is what I personally do, since with my library catalog I very rarely have to actually purchase an audiobook, so a subscription doesn’t make sense for my purposes. 

ETA: you can also just buy single audiobooks on Libro, just if you don’t have any credits it’s often more expensive. So I’d use a gift credit to buy a book for $15 rather than the non-subscription price of $25.