That stretch is fine: it just takes traffic to and from the peninsula.
No hills: a few metres of 5% as you pass Boscawen-un Stone Circle (car park).
https://ridewithgps.com/routes/48363019
Definitely not worth going left or right flanking if you have 1000 miles to go. But the ride round towards St Ives is lovely if time is not of the essence. I dive off the A30 into Newlyn (R @ x (B3317)) rather than going round the bypass.
As for beyond Penzance, I've gone both ways: via Marazion or up and over to Hayle (A30). If the latter worth going via Gulval (L @ x imm after Jewsons) to minimise time on A30 (cycle lane for much of it).
Via Marazion takes you to Redruth, otherwise via Hayle minor roads through and over the Downs to Camborne and then Redruth. After that, the choice is across to Truro or up towards Newquay.
Personally I prefer the Marazion, Praze road to Redruth. Several other variations available (have done most).
No idea where
@toffee's "killer hill" is. Riding TO LE you climb 70m in 4km (via Newlyn). A nice gentle warm up first thing.