The best value for money is AIA but unfortunately they don’t offer cover to DTV holders, next best one is AXA, you can apply and buy directly online, no need to go through a broker.
AIA is the best for long term health insurance (more than 1 year), it’s the biggest insurance provider in Thailand for Thais, and as the saying goes, when in Rome do as the Romans do. Contact
When you apply for the Thailand Pass the start date of your insurance policy will be before your arrival date, this will likely be flagged when you apply. If you have a single trip travel policy (which most are) your policy has effectively expired. My advice would be to buy a new one from AXA.