Is it just visually / right next to each other?
Then decals might come in handy combined with one or two blocking volumes to have collision.
Otherwise would it be acceptable to do it not exactly at the same time? You could add a “spawned tiles per second” number. Divide it by “Delta Seconds” from “Event Tick” / “World Delta Seconds”, round up, pass that number to your recursive function / make a for loop out of it and just run it that often. Then the next frame You go on until the condition for your recursive function is met?
Like this you stretch it out and reduce the impact on FPS / game flow for the trade off that they don’t spawn at the exact same time but over this time of half a second or whatever.