Weighted Ejector is a block that launches items to the air. However, for some reason, launched items don't teleport when going through event horizon of Stargate. They just ignore it

Not sure how exactly the Ejector works, but I assume the reason for that is that it's just an animation and there are no physical item entities involved, in which case, there would be nothing I can do.

Though, if you don't mind, would you confirm/deny my hypothesis by turning on hitboxes (F3 + B) in-game and checking if the items being launched have hitboxes? (I would normally check myself, but I barely know anything about Create so just setting up the necessary machinery would be hard)

Yeah, they don't have hitboxes... Welp, I guess it'd be hard to fix then with mixins and stuff (searching where to inject, etc.)

Yup, fixing that will require directly making the Stargate into some kind of Create component I suppose, which I don't know how hard that is. We'll see...

Nope, I think you'd need to mixin somewhere around here:
https://github.com/Creators-of-Create/Create/blob/da9f15cbdf4be8468f585f1f30b364d8cc4a652c/src/main/java/com/simibubi/create/content/logistics/depot/EjectorBlockEntity.java#L355
Virtual item is doing raycasting to check for obstacles so you need to actually be an obstacle for it.
Or you need to mixin into method above it to perform actual teleportation before or after any check for obstacles.
Still, minor thing