If we want to maintain scarcity and fairness here is a solution: Use a system similar to the airdrop system for promo cards. Pre-orders can be taken up until general sale. This would work out as:

x packs guarantees a promo card
remaining promo cards are assigned via lottery up to the total mint

RW example:
300 packs guarantees a promo (if all packs sell out then there will be none to allocate via lottery, but not everyone will buy in batches of 300).

Modest blowout:
750,000 packs are sold, this is 25% of all packs and a maximum of 2500 promo cards, (its likely to be more like 2000)
remaining 7500-8000 are allocated to pre-order participants via a lottery 1 pack = 1 chance.

Full blowout:
3,000,000 packs are sold, this is all available packs and a maximum of 10000 promo cards (its likely to be more like 6-8000)
remaining 0-4000 are allocated to pre-order participants via a lottery 1 pack = 1 chance.

If all packs are sold in pre-sale then the total number of packs was too low.

I also think anyone who purchases during the pre-sale should gain equal access to the airdropped cards