Hey dont feel bad a lot of builders take the LOWEST PRICE from the subcontractor bidding the jobs.
Did you bother to ask if your plumber had a Masters license?
Was the Master on the job site at all?
Luckily all is not lost you have viable options.
1- Install a back water valve (check just after the main trap) to make sure city sewerage doesn’t come back to your home
The only draw back is water seeks its own level AND if the city sewer is blocked you cannot use any fixtures in your basement if applicable.
If you did use the bathroom above the check the pressure ( . 434 height) may force the check open so your sewerage is forced into the city system.
2- You can go for a great sewerage ejector like Federal or the new Bell and Gossett
3- You can contact the builder/ plumber/ City and ask about the possibility of having your sewer raised slightly so the invert into the city sewer is higher the city’s sewer crown