Options for Niagara Falls to YYZ?

Whats wrong with GO TRAIN?

According to their trip planner, I need to take 3 different buses to get to the airport. That's not very ideal for someone with luggage/out of town visiter

NF to Toronto = Car Service - Bus - Train - Rental Car

As you've discovered Car Service is crazy expensive.
A Rental Car is much cheaper, and direct.

Bus (GO - Greyhound - MegaBus) & Train (Via Rail / Amtrak) both go to Downtown TO
Then one has to make their way to YYZ Pearson
Union-Pearson Express Train works well for this

Alternatively, one could make arrangements to get off the Train at Oakville (beleve it is a "flagged stop" / so by demand only)
And then make one's way by cab to YYZ Pearson

Via Rail is a comfy choice for this trip
And it along with UP Express easily works with Travellers who have luggage
The downside tho is cost & convenience (limited schedule)

Therefore a Bus & UP Express may be a better combo

Make sure you leave room if you plan on taking the via rail / Amtrak train it's coming from nyc and has highly likelihood of being delayed. Unless this have improved but I doubt it

Tai Pan busses are cheap because the Casino sponsor it and want people to come gamble and put their money on the tables. They even giving them free $10 chip to start playing.

I remember seeing some small 10-18 passenger bus pick up people from Brantford, Grimsby etc to YYZ.
Not sure how much are they, OP can do a research about it.

uber to Hamilton GO station, GO to Union , UP express to YYZ - maybe thats an option ?

I don't think casino buses do 1 way towards GTA.

I think it's GTA based round trip only, same day return, min 5 hour stay. I think you also need min age 19, no luggage allowed.

Can you offer to drive them to the airport?