Have to put a plug in for Antiques & Collectibles Insurance Group.
https://aciginsurance.com/
I compared them with CIS above when I got my insurance and ACIG was much more cost effective at the time but it was many years ago now and I haven't looked at pricing recently.
I've never filed a claim so can't vouch for that process but every interaction I've had with ACIG to discuss anything has been very professional. They cover the value of items in transit both to and from you, so I often get auction houses to reduce the "shipping & insurance" costs for things they send me. This makes this insurance even more cost effective at times.
Regardless of the company, make sure you read the terms & conditions very carefully! Deductibles, exclusions, etc. differ wildly.
jeff