That assumes that new routes would feed the SWMain line - they could be operated as extensions of the GW main line !
But agree that there needs to be a decent amount of upgrading to the SW mainline from Basingstoke.
The issue with the GWML option is there's not much scope to connect to London and either you're reversing at Exeter or missing Exeter (assuming you're coming in from that end, which would make the most sense) or you're not increasing capacity to Plymouth if you come in from the other end.
SWML had 8tph paths up for grabs post Crossrail 2 from Waterloo to places beyond Woking, so that's means that it's much easier connect to London.
Yes, it'll be quite expensive to redouble the WofE line, but it could be fairly useful in many ways.
For example, there's limited paths through Southampton and so a second Weymouth to Waterloo service could run via Yeovil. That could be formed of 5+5 units with half going to Plymouth and half to Weymouth.
To get a decent journey time to Waterloo it could be Yeovil, Salisbury, Woking, Waterloo.
For Weymouth, it would then call at Yeovil Pen Mill, Dorchester West and Weymouth to give it a fairly decent journey time.
For Plymouth, it'll call Crewkerne, Axminster, Honiton, Exeter Central, Exeter St David's, Crediton, Okehampton, Tavistock, Devonport and Plymouth.
At Exeter, I'd be tempted to run the all stops services from Plymouth to Crediton and from Barnstaple to Crediton where they then pair up calling at all stops to Exeter Central where they split with one portion calling at all stops to Exmouth and the other all stops to Axminster.
I'd also be tempted to extend the current WofE line service to Exeter to Barnstaple calling at Newton St Cyres, Crediton, Copplestone, Eggesford, Umberleigh and Barnstaple.
Likewise, I'd extend the Exeter semi fasts to Paignton calling at all stops.
That would give a lot of stops 2tph or better in and around Exeter, it would also increase the number of stations with direct access to a London and increase the number of services to London for some more.