St Austell eh?
Many folk call it St Awful ........... but that's another subject.
St Austell to Liskeard is generally along the A390 via Lostwithiel. It's a bit busy and a bit lumpy so consequently is a bit difficult.
North out of St Austell is a long way up hill so it would be less of a trouble staying on the A390.
Where do you aim to be next?
My advice - as seeing you are in St Austell - is to stay south and head for Tavistock A390, then turn north up the A386 for Okehampton.
You could try to tackle Dartmoor and over to Moretonhampstead, but it is very lumpy up there, and there's a horrible long climb up Pork Hill onto Dartmoor itself. Not recommended IMHO.
If you want a scenic route, turn off the A390 east of Lostwithiel just after East Taphouse and cross the A38 at Doublebois. Head north there via Minnions and Upton Cross to follow the B3254 to Launceston, then the Old A30 to Okehampton through Lifton, Portgate, Lewdown and Sourton.
Mick F. Cornwall