Ally,
As you say Trevose is the better course and there are also the recent M&E alterations to view.
As to the Holywell, holes 1-3 and 13-18 play over some lovely humpy-bumpy land, where some of the original main course was I believe, with quite a bit of quirk as well. The par-3 15th is particularly fine.
Given that there’s plenty of daylight in late May in Cornwall you could probably add the Holywell onto one of your other play days.
As well as the main course Trevose also has two separate nine-hole courses, which I think may have been upgraded somewhat.
Whatever you decide have a great time.
There’s also Newquay, which hasn’t had much comment on here but I’ve heard it surprisingly good in a quirky way.
Atb