Snow 1964 - the problem is simple. The DfT will not allow GWR to have “enough” units in your eyes or “enough” staff or materials to maintain those extra units.
You could give Bristol or Exeter all the units you want to give them but if they haven’t got the bits or the staff to maintain them, then the depots will just get full of unserviceable units.
Ideally, GWR should be working at 80-85% availability levels, by class, for its fleet of 15x & 16x. Reading did work at 90% when it had the Turbos but 2/3 of that fleet saw Reading each night. West is more spread out and what goes to an outstation at night, comes back out in the morning, short or not. A unit can also go for a fair while without seeing the Marsh so any unit issue can reverberate through the service for a fair few days.
You could dedicate units more to route and pull them into the Marsh more often but that needs more stock and staff to make it work. The money for that isn’t currently forthcoming but GWR are interested in the Welsh 158 units, when they finally get released, some to boost formations.