I'd be very curious if it is indeed possible to bank time at this point, as that would reduce your effective cost for the next 3 years of game pass ultimate to ~R55/month (suppose that should probably be inflation-adjusted but I'm feeling lazy)
It should be noted that those vouchers are going out to all members of the Friends and Family plan, not just the primary account holder - i.e. to excerpt
the page: "We’ll be sending Xbox Game Pass Ultimate codes ... to both Primary and Secondary members ... All participants in your group plan will also be receiving the same personalised offer, so that you can continue playing together."
i.e. if you can bank time at this point (I'm unsure), that's a route to 3 years ultimate for each of the current members of your plan - I'm not quite sure how that differs from "have 5 people use the same gold and game pass" as I think each of those people already have separate xbox accounts. They'd simply need to redeem the voucher for their individual account. You'd have to do something a little over 3 years from now, but if it reduces your cost over 2/3 for that duration it'd seem like it'd be worth it.