First, I'm not a pro plumber, just a long-time DIYer.
If you're having root problems, the good news is that they simply can be "roto-rootered" out to open up the clogged line. The bad news is that the roots will grow back in 1.5-2 years, creating the same problem over and over again.
To open that corroded clean-out, I would apply the old "red wrench" (aka a torch) to really heat it up, and then turn it out with a large pipe wrench with a cheater pipe on the handle for leverage.
The only permanent solution for a root problem is to replace the main drain line sectional cast iron with "glued" white plastic PVC or black plastic ABS, if allowed by local code. The "glued" joints actually are chemically "welded" and will not allow any roots to enter. Anything else will be a band-aid.
Get three bids and CHECK REFERENCES. The low bid isn't always the best.