I do wonder though if you could take out the stops at St Erth, Camborne, Redruth, Truro. St Austell, Par, Bodmin Parkway and Liskeard as these are also stopped at by the Penzance to Cardiff Central (2U20) service 30 minutes later.
What’s the point of running it beyond Plymouth at all then?? That only services Penzance customers solely. Truro is a ridiculous stop to omit, busiest in Cornwall by far - not just because of Falmouth connections in Summer, but actually mainly due to being the principal town in Cornwall. Everything should stop there (and currently does) and Bodmin Parkway gets very busy indeed also.
You could get away with only stopping London services at Par that connect with the Newquay, and those could in turn omit St Austell, and Camborne could be reduced to Regional services only - already several GW and XC skip it. A couple of peak trains could skip Redruth but I wouldn’t recommend many, and in summer St Erth for the St Ives Bay Line is rammed. I wouldn’t reduce Liskeard or Bodmin Parkway either, except on winter Sundays when Looe is barely running, and then you can skip the former then by all means.
1tp2h from London calling Liskeard, Bodmin Pkwy, Par, Truro, Redruth, St Erth, Penzance - connecting with the Newquay service
1tp2h from London calling Liskeard (not winter Sundays), Bodmin Pkwy, St Austell, Truro, St Erth, Penzance
and then the hourly Regional service calling Saltash, St Germans, Liskeard, Bodmin Pkwy, Lostwithiel, Par, St Austell, Truro, Redruth, Camborne, Hayle, St Erth, Penzance.
4tpd Plymouth, Devonport, St Budeaux Ferry Road, Saltash, St Germans, Menheniot, Liskeard peak shuttle will suffice for the awkward stations