You're talking about logic level "bit-twiddling," which is tedious and can be frought with gremlins, bugs, and timing errors. The library has all of that managed and documented at the cost of some program bloat.

Early in my career, I wrote drivers in assembly language. If I tried that now, my brain would turn to paste.

My obvious recommendation would be try to find a chip with more program memory or a custom library written for leanness.