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