Can members buy a new collection a few days before everyone else?

Yes. In Plugins → Membership, open Member-only products and click + Add product:

1. Product: pick the product. One rule per product.
2. Access type: Early access — members first.
3. Required plan: leave it on Any active membership, or pick one tier.
4. Available from: the date everyone else can buy it.
5. Click Add rule.

Members can buy straight away. Everyone else has to wait until that date. If you want a product to stay members-only for good, choose Exclusive — members only instead.