The services are well-used from Plymouth, but for very few actual through journeys. If it was a 5 car GWR IET running in its place, it would have the same passenger numbers
Arguably more, as there would be more seats.
If there was suitable capacity you'd want the XC service to start south of Exeter and/or the GWR service to overlap to the north by at least to limit infrastructure issues at Exeter (generally trains which terminate at a station use up far more platform capacity than through trains) and to reduce the number of passengers who would have to change trains.
This hourly XC service is Manchester Piccadilly - Stockport - Macclesfield - Stoke-on-Trent - Stafford - Wolverhampton - Birmingham New Street - Birmingham International (Airport) - Coventry - Leamington Spa - Banbury - Oxford - Reading - Basingstoke - Winchester - Southampton Airport Parkway - Southampton Central - Brockenhurst New Forest also for Lymington and the Isle of Wight - Bournemouth
so this needs to continue to run with this list of station calls for all these connections.
This hourly XC service connects Oxford and Reading, not just Southampton and Bournemouth, with Stoke-on-Trent and Manchester. It provides very useful connections from both North and South to Coventry and Birmingham Airport.
Rail passengers using these train services will not want to change at Birmingham New Street.
Birmingham New Street is a very busy station and may not have the platform capacity to split either the XC Manchester-Bournemouth or Manchester-Bristol service into two separate services at Birmingham New Street
It's a shame that not more electrification has been delivered between Birmingham and Basingstoke, as with it there that could then allow an 8 coach 350 with batteries to run the service between Manchester and Reading comfortably (rather than it being rather tight with the current extent of electrification).
At Reading:
Voyager arrives from Bournemouth xx:06 (platform 1, 2 or 3) goes onto form
Voyager departure to Oxford xx:26 (arriving Oxford xx:50)
Pair of Voyagers arrives from Newcastle xx:09 (not platforms 1, 2, 3 or 7) 0.5tph frequency,(in the other hours it heads to sidings) where it goes on to form:
Voyager Departs Reading 00:15 to Southampton
350 Departs Reading xx:16 (platform 7) to Manchester
350 arrives from Manchester xx:40 (platform 7)
Voyager arrives from Southampton xx:45 (0.5tph, in the other hours it's from sidings) which then becomes:
Voyager Departs Reading xx:51 for Newcastle
Voyager arrives from Oxford 11:44 (having left Oxford xx:19) goes on to form:
Voyager Departs Reading xx:52 (platforms 1, 2 or 3) for Bournemouth (5 coach 221)
By running to Oxford you maintain the direct service for those who wish to use it (and a guaranteed connection to Oxford if the train is late) however you also give those willing to change the same journey time.
Other than the Exeter semi fasts (which only run every other hour) there no other trains between the arrival of the class 350's and their departure on platform 7, so you'd only either need to find another platform for the Exeter semi fasts or send the 350's to a siding every other hour.
The long time that the Newcastle service is at Reading (because it's not going to Southampton is no different to now where it sits in platform 3 - however that could be an issue if by using the 350's to release extra sets it could be regularly run as a pair of units, especially as the Bournemouth to Oxford service (5 coach set) would be a good fit for using platform 3.