only unis still with vacancies will participate in subsequent offer rounds.
This.
Majority of offers are made at round 1, and as the rounds go on, the chance of being offered one is reduced. Is the course you have applied for CSP? If so, the places in the course are capped due to the limited number of subsidised places available. If you meet all the requirements for the degree you have put in the preference, I do like your chances as you're probably right at the top of the selection order for round 2... but it will then be entirely dependent on whether the uni participates in round 2 (they won't for this particular course if all places have been filled & nobody withdraws).
I'd definitely be considering a backup plan just in case - definitely accept the offer you got in round 1 though. Don't let it lapse or reject it as you may end up with nothing in the end. You can withdraw from it later on if you decide to accept another course.