skip the detour via Garrigill
And it's a s_d of a climb back out of Garrigill, even the road version.
I've only attempted the off road section from Garrigill to the B6277 once......
The on road section from the B6277 to Nenthead is very steep. I've fallen off at least twice going uphill (ice under back wheel > twiddle, twiddle, bang). Even steeper going down. But it is a lovely scenic road.
The off-road version is a much easier tarmac climb to Priorsdale, and the long off-road downhill should be fine on a gravel bike or strong tourer. Again, lovely remote moorland.
Both nice routes, so decide on the day
I have done the climb up over hartside by the difficult route 3 times with luggage and enjoyed every one. Tough but very rewarding imo.
I've never tried this route - did you need to do much walking?