Another fix, but the most expensive, is to have the sewer line replaced with polyethylene (PE) sewer pipe. PE pipe may be installed by the “trenchless” method, a really neat and ingenious technique that involves some expensive hydraulic equipment to pull PE pipe through already laid clay or cast iron sewer lines. The equipment is expensive (so you can’t DYI), but a sewer contracter generally charges about 30-50% less for a “trenchless” sewer line replacement.
The advantages are that the PE pipe run has fewer fittings (in many cases there will be a single continuous run of pipe), and the chances for re-invasion by roots is correspondingly less. Roots grow into pipe (especially clay) because of leaks that attract the roots. A continuous unbroken run of pipe will not leak…
PE also has a much smoother bore than cast iron, and for that reason drains better.
I no longer do major sewer work, and limit my work in that area to short sections of tie-in and retrofitting. (My back isn’t what it used to be 25 years ago). But I’ve had the chance to see the trenchless sewer guys in action on a few jobs, and I was very impressed.