The proposed schedule is *NOT* fast. Prototypes coming in 2027 and 2028. Production aircraft and IOC after that. For a plane that already exists.
Not saying if this is the case here, but there are explanation for the what seems to be such a "long" schedule to delivery of an "off the shelf" system.
The P-8A example.
The delivery schedule first two aircraft is when the the frames are officially transfered to the Air Force. This may not be the same as the Air Force getting their hands on the aircraft and start tinkering with it.
With the P-8A, the first two flying frames were not "delivered" to the Navy until long after some production frames were handed over.
During this time they were making all sorts of changes to the planes as they test out new increments of upgrades which ultimately got incorporated in to production frames. Having changes made while the frames still under Boeing control allows the changes to be incorporated under a modification configuration as opposed to a retrofit configuration. I suspect the way the contract works, modification to an undelivered frame is much simpler than retrofit.
Modification contracts may also have different buckets of money to work from, as opposed to retrofits. But that aspect I am not familiar with.
As for the first two E-7A frames, I understand that it will be the UK configuration. That make sense as all the long lead planning would have already been done for the original UK proposal before they dropped 2 frames.
The Air Force would then upgrade the mission equipment to open architecture, similar to what the latest P-8A configuration is flying except with Air Force specific requirements. The "tinkering" will probably be done with a set up they already have in mind.
All this is speculation of course.