Not done it but believe the start is a no-go hike/shoulder bit.
Reckon on starting @ Newton Stewart would be best plan.
1- There is still quite a bit of Snow
2- Current forecasts are lots of rain = Boggy.
3- Lots of hike a bike from Newton Stewart to Moffat. Think 1hr hike up from the Daer res to the top of the hill on a peat/tussocky bog, covered in snow!! Same from Sanquhar and again the section from Wanlockhead radar station is a nightmare imo.
Bit from Moniave to Sanquhar = eugh!!!
Once you get to Moffat it is plain sailing
Yes it has all been done, many yrs ago for mbuk. Know some guys who have tried it but gave up after 2 days!!! Not much in the way of accom en route so wild camping makes it even harder????
I sometimes do a route from Sanquhar to Dumfries by getting the train up from Dfs and biking back. Had thought about it last weekend but looked like still too much snow on hill tops