I understand the storage and maintenance issues that you may have with a fleet of equipment, but maybe in your current situation 1 piece of equipment just is not up to the tasks that you need from it. Maybe if you bump up to 2 pieces of equipment being a B series (if that fits your loader and backhoe needs) and a zero turn mower you wouldn't be increasing to a fleet and have better luck getting all your chores done.This and it's all in one machine with multiple attachments vs having a fleet of different machines and a place to store them all including the additional maintenance of each machine.
It appears that a lot of the Kubota range can have a bagger attachment for it. Then you can have it attached permanently and the space a zero turn takes up is only slightly more than a backhoe that is being stored or a bagger attachment for a B/BX series.
The added maintenance cannot be avoided if you get a zero turn but it may turn out to be the same or close to the same time commitment as the single bx because you are splitting the time between the machines.