If you extend the group buy 2-3 months though, people that signed up early may lose interest, and I guarantee you will have orders cancelled when it comes time to pay. By the time the orders ship, people will have changed addresses, it will be a mess. Just look at gh60 gb.
If we're stating opinions, my vote is to stick to the initial deadline. If we make MOQ, great. If not, try it again later when a bunch of other GBs are not running. If granite and olivetti don't make it this round, do them later. Maybe even do only one color per GB to make it simpler.
There are also people who wait until the end to order. If you don't keep the deadline firm, they may not commit.