[john70t]

Get a rootorooter and a camera inspection done for a few hundred bucks.
Cast iron pipe will de-laminate inside and curl inwards. That might fix it for a while.
There might be a break which is filling up with tree roots, or a sag/settling in the pipe.

There are 50ft snakes at the box stores or probably some type of power flushing procedure, but the problem will probably return.
The washing machine and shower tub dump should be able to flush normal debris I would think.

A new main to the street might be $7-15k depending. Spring or fall to save any plants in the front yard.

A new high flow toilet is probably a bandaid. Sorry to bear bad news.