Never ridden the places you mention but did manage to pull in both the Marin and the Penmachno a couple of months ago.
On a nice sunny day the Marin is excellent in my opinion, we really enjoyed the route, there is some fire road climbing but it’s worth it.
The Penmachno we did on a howler of a day, cold and serious sideways rain had us caught out a tad, and even though only 31k it had us a tad defeated on the open moorland section. On a nice day (or even non sideways rain day) I imagine it would be ace, the views are supposed to be great, not that we could see anything.
Enjoy