I'd use the auger when needed, after pouring down a dose of root killer and letting it try to do the job.
I'd also try to discern which tree or shrub is the source of the roots and decide if keeping it around is worth the trouble. (Mine was a lopsided tree, and it wasn't.)

The roots are going to continue to grow. Unless you cut the offending tree(it will still grow roots for up to 2 years) or dig the pipe, cut the root and fix the hole where it entered, you need to auger every year at least.

Root killer down the piep won't do alot since it will just run through the pipe to the street.

camera was rotated so image was upside down. roots where hanging "up". looked funny till we figured out the image. the main line goes under the footing. city sewer was added after house was built. house was built with well/septic and than 5yrs later city sewer/water was added. just love paying for that stuff twice. i planted shrubs in front 15yrs ago? no trees. L- shaped shrub line. so there are 4 bushes parallel to line. directly on top. next to 3yr old new sidewalk.