I found a connection that would save me several hundred dollars to build my own connection with two plane tickets. First flying on Delta and then onward on JetBlue to reach my destination. I know its a good idea to plan extra time for the DIY connection. Is there also insurance that'd cover rebooking the second flight if I miss it due to a major delay on the first leg?
Yes and no. There are many companies that are happy to sell you travel insurance. However all of these have different "terms and condition" that describe in excruciating detail what is covered and what is not. They can be very hard to read and understand and they typically include a long list of "exclusions" and "exceptions". The insurance companies want to make money and do they primarily insure against cases that are rare or unlikely to happen.
Here is a typical example: https://www.allianztravelinsurance.com/product-details#/International/1 . Coverage for missed connection is stated as
"Reimburses expenses resulting from a covered delay that causes you to miss your scheduled flight or cruise."
But it only covers up to $800 and it's vague on what a "covered" delay is. The only way to find out is by carefully studying the terms and conditions during check out.