I would happily pay to fix the start time I want - which wouldn’t be 5/6am, but was a mildly competitive slot.
This would be my suggestion if you want to "milk" the people who want to have a particular slot.
Registration as before, a "priority" for Audax UK members / LRM riders / people who prepaid and after say two weeks open for everyone.
Then, when registering, you get to to see the available timeslots and how many have registered so far in each slot. With the big disclaimer that if a slot is over-allocated there will be a "lottery" and you may end up in a completely different slot. As before.
The new feature would be that at this point, and this point only, you get the option to buy "insurance" that will guarantee your chosen timeslot.
A timeslot that is fully filled with "insured" riders should be shown as such and not selectable.
Changing to a different timeslot at a later time should move the insurance as well.
It should not be possible to buy insurance later ("oh gosh, my slot is popular" -> tough luck, you're in for the lottery).
It would be a choice to display the number of "insured" riders in each slot or not; I don't know what would maximise profits
Price of insurance? £25? £50?