A good frame of reference would be the VPS that lemmy.world is running on imo. Looks like they upgraded to a 4 core/16gb setup to handle the influx of users, so if your instance is running under 1k users, I believe those specs would be sufficient.
If it starts chugging, I wonder how well it’d work to run the server on the laptop and the DB on a VPS (or vice versa).