Keep in mind that you can totally worship several gods at once in FR. When we choose a deity that just means that its our primary deity.
Vlaakith probably won't mind if someone also worships Tiamat, plus I doubt that you have to publically declare your faith for it to count anyway.
Tiamat? Nope. I don't see Vlaakith minding that. Everyone else, like Selune for example? She would mind that. At least from what I remember when it comes to githyanki who tried to leave Vlaakith.
I'm all for more options, but those feel like nothing when your githyanki is locked into Vlaakith worship, even when they supposedly worship somebody else that Vlaakith would never approve of. You are already limited in your RP this way since you can't imagine yourself being an outsider because githyanki pc is written with Vlaakith and her githyanki culture in mind.
The tricky part is explaining how your githyankee even knows about Ilmater or Lolth? It's not like they are common knowledge in the Astral Plane.
I find it funny that you can be Lolth's priest as a non-drow. True, everyone can worship whatever diety they wish, but I thought that Lolth would only ever reply to you back if you were a drow. Usually a female drow, but male Lolth's clerics aren't unheard of either, although that tends to be very rare. Still, Lolth won't care about you if you aren't drow.
So, non-drow being Lolth's cleric is kinda odd in itself.
This being said, there is the question of how Larian deals with it, as this is a video game and most of the GM-ing and writing will be fixed. If Larian has already made assumptions and written them in (like the Vlaakith-following Githyankis), then we probably can't ask them to remove that. We may ask them to write also for a non-Vlaakith-following Githyanki. That would be great
They could just use already existing voice lines that other races use (like humans) which are neutral. Dialogue choices would need some writing though since those are written in mind as you being "true githyanki" who is apart of githyanki culture.
