I concur, the vent, lifting eye and stack are probably conducting some heat and that finds its way out between the factory installed insulation and the roof.
Take the roof off, and install some rock wool like you suggested or even regular fiberglass. Just be careful not to put too thick a piece in, compressed insulation is almost as bad as no insulation.
Make sure all is well caulked after reinstalling the roof, wet insulation is worse than no insulation.
When I was still building boilers I though of all kinds of way to isolate the lifting eyes from the water jacket. Most of em would add a considerable amount of farting around to finish the boiler. In the end I didn’t change anything, it was already a conventional with much higher stack temps than a gasser anyways.
On my waste oil boiler I have six inches of fiberglass on the outside, the lifting eye still gets wamer than ambient.