Mary T. - Sounds like you're moving in the right direction. After about 40 years of doing fingerpicking I've moved from thumb and four fingers down to thumb and first finger mainly.
That is, alternating bass with the thumb and first finger picking melody or running a pattern. You can also sort of switch that around and pick melody with the thumb and do alternating high notes with the finger/s (out of chords).

And yes, sometimes you can go pretty high up the strings with the thumb, and once in a while pretty low with the finger/s - depends on where you need to find the melody notes. Also you can alternate on the same string with thumb & forefinger to run fast melodies - the fingerpicker's version of flatpicking.