This achievement caps off at 750 right now, but I feel like it should go to 1000, because many of the other achievements go to 1000 or higher 'items donated' or whatever. Is this in the works for this Fireside, maybe? I just wonder whether I should continue drowning in all of these pumpkins or if I should chuck 'em.

We're probably not going to find out until Fireside starts (even if staff frequented this forum, that doesn't seem like the kind of thing they would outright confirm ahead of time) but if nothing else, Fantine and/or Charity usually have new items available in exchange for donations, so you could hold onto your leftover pumpkins for a couple weeks to find out if you need any of those items, even if the achievement doesn't get a new tier

Staff has said in the past they are moving away from grinding Achievements. And then sometimes they add tiers to them anyways. We users don't know if more tiers will be added.

Many users who don't need the Achievements any more still mass donate. It's often possible by the end of Fireside to buy from the new items that NPC Fantine or NPC Charity may give. This price is usually lower in my mind than the time-cost of doing the donations in the first place.

Fragmentize them, donate them to the Donation Center or chuck them if you like. It depends on how frugal you are with your sP compared to your RL time.