Will adding my own domain end my free trial early?

Only if you are actually buying it. A paid add-on makes you a paying customer from that day, with no order cap and no end date — and the page warns you before you ask, not afterwards. Waiting until the trial finishes and adding it then costs exactly the same, so there is no rush. If your plan already includes a custom domain, nothing is being bought and your trial runs to its own date as before; in that case the page also says the other half honestly — the trial can still lapse, and if it does your site pauses on your own domain just as it would on ours.