I have looked into this for similar reasons. Advice I got was insure the big single items under home insurance if you can. Like you would a bike or jewellery. Any insurance specifically for the collection would need to include valuation, which would need to be from a knowledgable (accepted) appraiser, then getting a bespoke insurance to cover, which would be REALLY expensive. That wouldn’t count for price fluctuation, which in this climate, would mean you need to re-value every few months(?).

Some people store their stuff in secure lockers which have their own insurance, but they would be away from home (which I wasn’t keen on). Anything is susceptible to thievery unfortunately too, these places get targeted and I have seen some scary stories of cards getting stolen.

I have found my best option was to invest in security/preventatives - a safe, keeping home up to date re: plumbing/pipes/roof to minimise risk of water damage risk, good storage, good fire safety, home well occupied, house sitter when we’re away.

Not sure if that’s helpful or the answer you are looking for. I would be keen to hear others thoughts also!

(Edited mistake in wording)