The keystone portals are based upon the nether star, that is awfully hard to get. I didn’t see them at the shop either.
I am wondering whether a wither might be trapped in a configuration with 5 shulker boxes instead of bedrock, but then I still need three wither skeleton skulls …
My first suggestion is to make it much simpler: keep the four gold drops in the corner, and replace the middle cross with ender pearls only. My second suggestion is to make a public portal at the community hall, so that everyone has access to at least one portal.
Apologies to @bluekelp who created the current setup, but it really would be better to keep things simple !
Why make it hard ? You want players to use the key portals, so that they can go to places … For example, I discovered this special place with peaks up to height 256, which looks like a great place to fly around in. I was thinking about setting up a keystone portal here but given the requirements this is actually impossible.
PM. Referred to links are: