I think what I would do is this.

Split the remaining Alexander 300 between Hartlepool and Stockton depot. Hartlepool can use them on the 1 6/7 routes and Stockton on the 10/12/13/14 and 39 and cover on the 36/37/38 services.

Move all remaining darts to Hartlepool as well. Any darts at shields would be swapped with enviro 200 making it easier for Hartlepool and Shields to mostly have one type of midi bus.

This would then be the order for the north East

13 Enviro 200 MMC
3 For Hartlepool
5 For Newcastle
5 For Stockton.

10 Enviro 400 MMC
5 Slatyford
5 Walkergate
Older enviro 400 where possible split between remaining depots.

This should not just help with CAZ but also see off some of the Manviro 200s which are very unreliable.