As far as I have gone through troubles of brakes and seeking solutions online, there's no way an expert can identify the root cause of such problems sitting behind a computer. The user actually has to do hit and trial, post the results and then one can advise way forward.
But before i say anything, mind you- your mechanic is just looking for you to be caught up in a cycle that only he can fix. So that "kit" supposedly costing 7000 should be ignored for the moment. (We all know he is referring to some kabli distribution block)
When you apply brakes, the weight of the car does shift forward so that doesnt mean the rear brakes are not working. That is the same reason brake force is distributed 70:30% split between front and rear. More weight on front (due to engine and resulting weight shift) can take more brake force without letting the tyre slip.
To be sure of if the rear wheels are not locking, on a wide, traffic-free carpetted road, drive the car to 60 and literally slam the brakes while keeping the steering straight as possible. Stop and come out of car. Observe the tyre pattern on road, the rear wheel skid pattern should be visible and overlapping on the front ones.
Now the diagnosis. How long have you kept the car with you?? How many times the pads/leathers have been replaced. How many times the disks, drums have been faced on lathe machine?? When was the last time you completely bled brake fluid??
If you are sure the disks, pads, drums, leathers are in good nick, (visible inspection), move on to observing brake lines at the wheels. They may swell during operation. Observe hardened rubber and signs of deformity. Next replace the brake fluid completely. Not just at the hubs only using two/three bleed strokes. Bleed as much that the fluid should be fresh from resorvior to distribution to hubs all the way. Would only cost you 2 bottles at max. The manufacturers recommend to replace brake fluid every 3 years as it absorbs moisture over time and starts rusting brake lines.
If still not working, one can further look in master cylinder seals, distrubution block, replacing disks, re-furbishing drums etc...... but dont just throw parts at the moment. Try to isolate the problem step by step.