It might be a few hundred to scope the main sewer line but you could probably find promos to lower the cost. There’s a “one stop shop” in my area that’s advertising scopes for $30, but they do plumbing, electrical, and HVAC and I haven’t heard of them doing any of those particularly well. If you find a company like that just be ready to put up with their sales pitch and then keep the video to run it by a different plumber for a dimebag or a case of beer or maybe even some real money.
Do you have many big trees on the property? That’s usually the biggest cause for backups like what you’re describing: roots sneak into the sewer line, then keep growing/expanding and catching all the toilet paper flowing by and then just turns into this big dumb obstruction that constipates the whole line
Which other companies come to mind? I’ll support them too!