Their note is $480,000 but the deed of trust secures up to $600,000

Underwrite against $600k. That's what the recorded instrument tells the world, and in most states a stated maximum principal amount plus a future advances clause is exactly the mechanism that lets later advances take the recording date of the original deed of trust rather than the date the money went out. Whether a given advance keeps that priority often turns on whether it was obligatory under the loan documents or optional, and states split on how they treat optional advances made after the senior has notice of your lien, so that's a question for a real estate attorney licensed where the property sits. Protective advances for taxes, insurance, and preserving the collateral usually get priority treatment on their own footing, sometimes above the stated cap.

Capitalized default interest and extension fees are the part people miss. At 24% on $480k you're adding roughly $9,600 a month, and nine months of that plus a couple of extension fees eats the $120k of headroom without the senior ever writing a new check. Your cushion is $780k ARV minus $600k, which is nothing once you subtract selling costs.

Things worth pushing for in the intercreditor: a hard cap on additional advances at a number you choose, notice of borrower default with a cure right, and a right to buy out the senior at par plus accrued. Senior hard money lenders sometimes give the notice and the buyout and almost never give the cap.

The other question is who funds draws four through six if the borrower stalls and the senior stops advancing. Your $120k is in the ground already, and finishing the job out of pocket may be the only way to protect it. Price that possibility, not just the interest rate, and get whatever the senior agrees to in the signed intercreditor rather than an email.