Anyone in my FC can execute buffs. We have more credits than we will ever use, so this makes more sense. I replenish them every 2 or 3 days to insure they are available. There is no fixed daily rotation on battle, gathering, or the crafting buffs. The majority of active members on determines what buff is most needed for the day. If someone would like one of the gathering or crafting type buffs they only need to ask in fc chat if anyone would mind. Usually my members don't mind if someone needs a different buff for an hour or two. When that person is finished they simply disable it and enable the regular buff again.
On days of raiding or reset day we work it the same way. Putting up Meat & Mead II, or Proper Care II for raiding, then when not in use it can be disabled and the normal buffs are put back.

Purchasing these buffs a bit different , but we've worked that out with veteran members rank are able to buy from the Quartermaster along with officers and leader.

Smaller FC's or new FCs might struggle to keep buffs active like this and a queue system might be helpful in these cases. It's been awhile since SE has done any improvements to FC management. It would be nice to have a QoL like this.