Also, don't know if there'd be any suitable single decks that aren't too thirsty but not too lightweight, but would going from double decks to single decks on the 35/X21/X22 not be an option?

Could do something like this with Ashington's E400MMCs:

Option 1:
19x E400MMC (72 plate) X14/X15/X18/X20
12x E400MMC (17 plate) Arriva DC - 7

Option 2:
12x E400MMC (72 plate) X14/X15/X18
1x E400MMC (72 plate) Ashinngton / Redcar float
6x E400MMC (72 plate) X93/X94
12x E400MMC (17 plate) Arriva DC - 7

Then maybe 7534-40 come North (given they've had a relatively charmed life in comparison to their Northumbria counterparts) and they cover the X20 + any decker capacity / spares needed at Ashington.