A two tier service, is what I propose, just as is provided on many other routes.
A two-tier service on other routes is usually a consequence of local traffic mixing with longer distance traffic - Milton Keynes to Euston or Reading to Paddington. This already exists along the entire Cardiff - Portsmouth route; with the direct trains being the nominal upper tier, and those services paralleled by Cardiff - Bristol, Bristol - Westbury, Salisbury - Southampton and Southampton - Portsmouth stopping services. Those flows largely match observation on where seat reservations are. A cursory check in genius mobile just now reveals that while there are certainly some long-distance reservations/counted places on currently running 1Fxx services; again the vast majority are within those shorter service groups.
I’m content to agree to disagree on this one, as I profess to only being able to see part of the story with reservation details - that won’t account for walk - up leisure travellers who may skew the balance back towards intercity - and that the info isn’t publicly available so I can only give my word as to the veracity of my observations.
In my book the Turbos “solve” the issue on the Portsmouth route. They could probably do with a tart up inside to a configuration akin to the 387s - commuter but 2+2 with a sprinkling of tables in the longer term - but the route is too busy for wingback armchairs and 4 luggage vans.
EDIT pressed submit too soon