First things first. Are you SURE what you see in lines is tree roots? Bio-mass gardually grows inside septic lines to the point it renders field lines usless so that line replacement in a new location is required. The stuff resembles tiny roots on a scope but easily distingused when pulled from line on snake. how long has it been since tanks were pumped? Are you sure pumper is finding ALL tanks. Many systems have up to 3 tanks. for now I'll assume this isn't bio-mass but if you decide that's what it is just say so and we can work on solutions.

If roots are between house and cleanout you can get more bang from root killer. At a time when little or no water will be dumped into lines, block line at cleanout with something similar to this. After line is blocked put your root killer in drain and use enough water to flush it down to and fill pipe above block. You might pour 5 gallons or until water backs up in tub drain. You can see that if killer just making contact with roots as it flows past roots as directed on product kills roots having killer stand in contact with roots for 2 or 3 hours should result in far more roots being killed. I wouldn't think it neccessary for killer to stand more than overnight. An alternate or additional remedy is putting killer in a root feeder similar to this and inject into soil near line in attempt to kill roots BEFORE they enter line. Killer lingers in soil whereas when used as directed on container killer briefly contacts roots then flushed out into field which isn't bad if you happen to have roots out there as well.

houzz wouldn't allow link to root feeder so just look for Ross Root Feeder at big box or garden center.