You could easily relieve the pressure on the 21 by reintroducing the old Low Fell shorts (can't recall what number it operated under when then Go Gateshead did it).
Bus runs every 20 from Newcastle to Harlow Green Bridge - turns under the bridge - and heads back to Newcastle. Takes pressure off longer 21 runs.
Though there'll need to be some forward planning for the major A1 upgrade at the Bowes to Team Valley section - that will drive increased traffic through Low Fell.
RE: New StreetDecks & ADL Enviro400s - X-lines fleet changes
Wistfully stuck in the 90s