Isn’t this connected with the inability to get the multi-mode 769s into service, which could cascade some more Turbo units from the Reading area?
Partly, but even if the whole Thames fleet moved over tomorrow, wouldn’t allow all the trains to become 5car and allow the Bristol area service improvements.
There are a few stations that have platforms of 120-135m in Wessex area, but there is a lack of money to add a few metres of platform extension and allow flexibility to run 6car units (even though a large chunk of turbo 165-166 fleet is 3 car). It is obviously much harder to form trains with a 2 and 3 car unit. There are even some 158s formed with extra driving car to bodge a 3car unit by splitting 2car units
You could also argue that not completing the electrification from Chippenham to Bristol also resulted in a lack of trains, some workings could have been 387s (although it did allow Heathrow Express services to utilise some 387s)
There doesn’t appear to be a strategy except convert some Wessex trains to Warminster and Frome to EMU operation, but only if Western decarbonisation phases 1 and 2 go ahead (which might be years if it ever happens), meanwhile Wessex continues with 30+ year old trains and no replacement planned.