What travel insurance options are available for people with a pre-existing condition such as diabetes?

This is one place where the law is specific. In Texas the seller must tell you about any pre-existing condition exclusion before you buy. It must tell you again in the fulfillment materials. You must also get the chance to learn more about the exclusion [1]. The same section covers the sales, advertising and marketing materials you were handed. They must be consistent with the policy itself, down to the forms and the rate filings [1]. Texas's insurance department puts it from your side. Policies may have exclusions for cancellations due to a pre-existing medical condition. Know exactly what a plan covers before you buy [2].

What the statute gives you is the information, not a list of questions [1]. The questions below are ours, not the statute's. Is there an exclusion? How does the policy define it? Is a waiver offered, and what does it take? Does it apply to cancelling, to care abroad, or to both? The two sources here do not define a look-back period. They do not set the terms of a waiver either [1] [2]. So we state neither: Don't know yet. The policy's own clause is the text that binds.

Then use the cancel window. It is the one clock the statute gives you. You may cancel for a full refund of the travel protection plan price. You lose that right once you start a covered trip or file a claim. The deadline is the 15th day after the fulfillment materials are delivered by United States mail. If they came by other means, it is the 10th day. A plan may specify a later date [1].

We do not name plans, and we do not say which one waives what. The statute quoted here prints no premium either [1], so this answer names no price for a waiver.

Read Tex. Ins. Code §3504.0006 at the link quoted here for what must be disclosed and for the free-look window, then read the policy's own pre-existing condition clause and its look-back period.