Looking for TRAVEL ADVICE please. UK Based.

I (64F) have an autoimmune issue (not yet officially diagnosed) which means that any time I get a cold/bug I am bed bound for a week or so and then get bad post viral symptoms that mean I am unable to function normally (very similar to ME/CFS or long Covid). Otherwise I function normally. 

For the third time in a year my husband and I have had to cancel holidays within a week of departure due to my contracting of an illness and not being fit to fly. Luckily, we have travel insurance but it is frustrating as in 2-4 weeks time I’ll be right as rain to go but we are unable to rearrange/rebook due to prohibitive cost of last minute booking. I cannot predict when I will get ill but try my best (as does my family) to avoid it and take all necessary precautions. 

We usually book holidays with flights direct from the airline and use Airbnb/similar as we require a kitchen and the peace and quiet, as hotels etc. do not offer coverage for dietary requirements and calm removed link

Does anyone have any advice on how to proceed with:

1. Rearranging the trip without it costing a fortune (it was long-awaited to the Caribbean)
2. If there is specialist insurance  to cover rearranging/rebooking in the future if required? 
3. If using specific travel agents would help if plans were to be overturned again in the future?
4. Any other tips for as much flexibility as possible.

Many thanks