Ummmm
This may be a rock and a hard place.
Let's pretend no line snaps occurred, but for some reason you were getting these 1536 syncs instead of the 2400 ones. This is to help explain the system.
When you moved to Max, during the 10-day training period the Maximum Stable Rate was determined - the highest speed the system found you could run at without excessive faults/drop-outs. From that the Fault Threshold Rate is calculated - 70% of MSR.
After that, BT will only respond to low sync rates if they are below the FTR.
Unfortunately, it is possible to sync above the MSR, and you may have been doing so in the past.
So, remember ignore the line breaks, with a sync of 1536 you would need to get your ISP to get your FTR info, to show whether or not a "proven" problem exists. If FTR is higher than 1536 - bingo - BT should fix it without argument. If FTR is lower than 1536, tough

.
Now - with the line breaks. The question is, as major work has been done - i.e. replacement of parts of your line - does the above argument go out of the window? Or should the repairs have given you as good a sync or even better? I don't know, but I feel it should be as good or better.
To help yourself, try resyncing at low noise times of day - such as before 9am. If you get a sync of 1728 or higher and hold it for 3 days without switching the modem/router off you will get a significant speed increase (about 400kbps). As soon as you have sync'ed please get your SNRM figure - reason explained later.
The other thing is to look inside your Master socket, (open it gently as there may be wires attached to it) - see John's earlier post and check only terminals 2 and 5 are connected. Terminal 3 should be disconnected. If you have extensions, disconnect T3 in those as well.
While you have the master socket open - assuming it is one with the divided face-plate shown in John's link, plug your modem/router into the Test socket that the face plate plugs into. See if you sync better and whether your SNRM changes. It should be very near 6, or very near 9. (6 is the default for Max, and the system ups it in 3s to cope with noisy lines. Your 6.8 could be an increase [good] because you sync'ed at a noisy time - getting 6, noise falls SNRM rises, or it could be a decrease [bad] from 9 as noise has increased since you sync'ed. Whichever, immediately after sync it should be close to the Target SNRM set for your line, so then we know how to interpret future figures).
That's as far as I can help I'm afraid. Oh, remember the BT ISP supplying your ADSL and that you deal with as a customer is BT Retail. The ADSL is actually supplied to them by BT Wholesale, who use the equipment owned and maintained by BT Openreach. All of course separate companies. Makes it nice and easy to get things done, as you have found

.