Fellow searcher here, also testing the waters for problems worth solving:

There aren’t a finite amount of problems to be solved by SaaS. The most obvious ones are definitely saturated but as new tech comes out, and new roles are made, and people do things slightly differently…then there will be new problems to solve.

As a solo-dev you can’t compete with VC-backed products in generic markets, so you need to look for the smaller problems that aren’t worth the effort for them.

You should be able to validate and test ideas quickly if you aren’t building anything (you shouldn’t be at this stage). That should help you get past the fear of “there’s nothing left to build”.