This process makes sense from a plumber that wants to make more money.
Putting a cleanout at the lowest point (curb) is not recommended in most cases. Lets say you have a backup and you open that cleanout, everything upstream is gonna come blasting out in your face. There's nothing wrong with where your your present cleanout is located. Snaking 75 feet is nothing. A typical snake is configured for 100 feet.
Call another plumber and get a video of your sewer line. Shouldn't be more than $200 and see what he says. If you considering having work done you should have a minimum of 3 bids. My last one I got six bids. Top bid was 48k!! the company I went with was 12k and performed high quality work. But you need to educate yourself and know exactly what the remedy is, or you may pay a huge expense for nothing.
Any company that's doing a bid will video your line (no cost) so they can properly determine what the issue is. Play dumb, ask questions, and try to gain as much info as possible before choosing a contractor. Some will pressure you to sign that day, tell them you have appointments for more bids.