here is my idea either in the Pf store or in the point shop you can buy something that extends the time you can be afk without being kicked for longer periods of time of course there will be a limit on how much upgrades you can buy but the can be small increments at a time like 1 minute or something if this gets added to the point shop this will add another useful aspect to voting hope you consider this -drama_llamma (if anyone has any ideas to make this better you can put it in the comments)
I feel like the AFK timer is something that shouldn't be changed. This is a farming server, not a sitting and doing nothing server. One idea I have that could provide the same endpoint as your suggestion is to have a plugin that would allow players to select an area and just apply one stage of growth to all crops in it. That way people aren't just sitting there AFKing, and they still get their crops grown. This could be sold by area (like claimblocks) in the pointshop like you suggested.
AFK timer will not be extended or added as an available purchasable add-on due to the simple reason of rule-breaking encouragement. We have AFK rules for a reason. The economy isn't something we're looking to cause any more harm.