I don’t think writing specs is the solution to your problem. Clearly, your “hobby projects” are things that you find fun. You write the fun parts but then avoid the not fun parts that would be necessary to complete something.
If you’re just “programming for fun” then good, you’re succeeding. I don’t think writing specs is fun.
If you really want to “finish” something, the best way isn’t to write a spec, it’s to not jump to another project when the fun factor dips.