I share your pain. Member Areas is nearly there, but right now there are a number of missing features. I've discussed these at length with the relevant teams but, as you'll see in my post, many remain.
I don't mention pre-selling in the post as this isn't something that came up in user interviews, but I see the issue for you. The only workaround that I can think of is to sell the next cohort as a Digital or Service Product in Squarespace Commerce so that anyone can buy it now and then gain access to the Member Area when it is available using a page password to sign up. A 100% discount code should prevent them having to pay again.
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