We had the same insurance + broker for our former 2019 C30 and the last renewal was $2240 (!). So $1500 for a 2017 C28 sounds roughly in line.
I have read elsewhere, probably Trawler Forum, about insurance costs skyrocketing for many folks for various reasons. Some factors seem to be inflation in general, providers wanting to change their mix of vessels insured, hurricane claims on the East Coast, and lots of new boaters with less experience getting boats during the pandemic. But it seems to vary a lot by location, model, and provider.
I'd call Bob Schwab at Anchor and see what options he suggests and maybe also call Global Marine.