David, what I suggest you do is cancel your subscription so that it won’t renew at the end of the subscription period. Then after it has expired, you will be prompted to subscribe again (or you can click the Subscribe button that will appear in the top right corner of the Hub when you have no subscription and no lifetime unlock), and then you can buy the lifetime unlock from in there.
Certainly I would advise waiting for a little while anyway as we will be running a sales promotion that will include the lifetime unlock option, as well as some savings on desktop Dorico, later in the summer. Even if you bought your annual subscription on the day of Dorico for iPad’s release at the end of July 2021, you still have at least a month remaining on the subscription, so it makes sense to wait at least a while longer.
(Though of course we very much appreciate anybody who chooses to buy the lifetime unlock at its current full price. However, I want to be transparent that a sale is coming in the relatively near future, since money is tight for everybody at the moment.)