Personally, I would get a whole new fan and shroud and start over. Pull the old one remove what sealant yo can, clean it very well with alcohol. Put butyl under the new flange and self leveling Dicor on that.
If you really want to keep that fan, then remove all loose sealant you can, clean well with alcohol, then re-seal with self leveling.
Someone keep me honest here, is it alcohol, mineral spirits, or what to clean the roof when you re-seal, it’s been a while for me.
Wash roof using Ajax rise well let dry completely, Tape all seams
Get a roller anð seat that tape, apply sealer over tape and every other place let dry
The vent razor all that shit off clean up with alcohol, remove black gasket clean w/ alcohol or replace, apply some filler compatible with fiberglass,
Dry, apply tape, roll it, cover with sealer
I hhaaattteee peeling off old lap sealant. Such a PITA.
An oscillating tool with a scraper blade works great to remove the old sealant.