One issue I would consider...I assume the supplier won't keep it on hand for you.
Do you have a building/ warehouse to unload/ store it for that length of time?
I never liked having material sitting on site for more than a day or so because of the moisture / humidity causing it to warp. (Mainly talking about dimensional lumber that will have drywall attached to it). It can be a PITA for the framer, drywall, trim guys that have to deal with it.
I'd ask the supplier to let you know when they get escalation notices & go from there.
Most of the time dimensional is on a slightly different timeline than the sheet stuff (OSB, plywood)