Drain issue- replace or not?

[Robin Evans]

Happy new year to all! We have a mixed use property that has one store front and 3 residential units, unit A, B and C. The store keeps having drain issue due to the age of the pipes and being flattened out. There's no incline for good drain flow. We have had it cleaned twice within half a year. We come up with two solutions. Solution 1: Plumbing company ran the camera and quote 12k to dig up and replace all underground pipes with PVC from the store to the main drain and all pipes outside to the main drain. Solution 2:  The store is 3 feet away from unit A.  Back door is facing the one bathroom wall of  unit A. We can open the wall, put a door there for the store to use and lock the bathrooms in the store. It will be for their exclusive use, not sharing with Unit A. There is another bathroom in unit A.  But unit A tenants (currently vacant) will have to pay for the water usage of that bathroom.  Only store staff uses the bathroom, not customers. Its a smalltown consignment store. So it will not be too much of burden for the tenants. This costs much less than replacing pipes. Any other creative thoughts on the matter? Thank you so much for any input!