At some point the via Old Oak Common option becomes available (though still a long wait) and that definitely wont be included in the ticket price*
* - Unless there are low HS2 usage figures and there is a push to encourage more connections at OOC on GWR routes.
If the direct Exeter to Manchester CrossCountry is not convenient, then changing at Bristol Temple Meads from/to the Bristol - Manchester is not that bad. Adds half an hour versus the fastest direct time, but because the Paigntons go via Weston, there is no option to save the full half hour. Southbound, the Manchester to Paignton train arrives about 10 minutes ahead of the train you would catch by changing at Temple Meads. I would go with CrossCountry, but maybe I am mad. London would be my second choice and Hereford 3rd.
All that said, if the timing worked, I would drive Exeter to Pontypool and take TfW from there.