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!

[Robin Evans]

@Bob Stevens @Nathan Grabau @James Wise Thank you very much for the input! 

@Richard F. @Nathan Gesner Thank you very much for the input! It is kind of a weird layout.The store and one efficiency are in Building 1. The other two units are in Building 2. In building 2, there is a breeze way between front vacant unit and its garage. Above the garage is a unit that has its own secondary drain line going to the main drain from the other side of the building (no connection with other units' lines) and has no issues. But the front unit's secondary line is tied to the same main drain line as the two units in Building 1. These three secondary lines merge into the main line at one spot and runs about 5 feet to a clean-out spot and then from there it goes on to the city drain. The city drain also has issues and they have to often clean it as well. We can see it takes a long while for plumber to run the snake and push the clog down further from the store. If the clog of the store is pushed forward to the main line to certain point and stay there for a while, the efficiency will be clogged too and it drains back to the store toilet. 

Camera saw there are three kinds of drain pipes, clay, cast iron and PVC under. And there are offsets in the clay. So liner is not an ideal solution.
We can see two newer concrete pads outside. Guessing the previous owners might have done some kind of repair but none did a complete replacement. At this point, we would like to get a reasonable quote and do it right so that we don't this headache in the long run.
Will shy away from this kind of older property in the future to avoid digging up the ground to fix drain that looks like spider web with patches. It does show a lot of pipe history, lol. The property will have better cash flow in a couple of years though.