Skyblock Builder Mod (1.18.18, 1.18.2) allows you to create custom skyblock islands using config. Simply create a schematic file with the structure block or Structure Saver from vanilla Minecraft and then set the possible spawn points.
This mod can be used server-side, but it can also be used client side to create skyblock worlds using the “Skyblock”, world type at “More World Options setting when creating new worlds. For servers, use “skyblockbuilder:custom_skyblock” (1.18+: “skyblockbuilder:skyblock”) as world type in server.properties or in forge-common.toml (config).