Electrical – Do future electrical works invalidate a satisfactory EICR

This gets seemingly odd because of the Article 680 rules specific to pools and other water features.

First off, pool feeders are not allowed to be direct buried as per 680.25 and the Exception thereto which AFAICT doesn't apply to outdoor cabling, so unless your AHJ disagrees with me and applies the exception, you'll have to dig up the existing UF and re-run it in conduit (either red brass RMC or IMC, or Schedule 80 PVC or RTRC-XW IF you protect the aboveground portions from physical damage).

Second, your pool will have an equipotential grid of some type associated with it (at least for the pool surround, if not under the pool itself) -- this is a Code requirement to avoid hazardous step potentials in the pool area. All conductive things directly associated with the pool (i.e. within 5' of it or part of the water circulating system) must be bonded to this grid as per 680.26 -- this includes the pool pump motor. From there, the pool equipotential grid is connected to the regular bonding system by the pool pump motor's EGC. While the AHJ can't make you put a direct link between the equipotential grid and the subpanel ground bus in, it's a good idea to do so, using the same 8AWG that's used for the equipotential grid.

Last but not least, your 8/2 should have a 10AWG ground -- 12AWG copper ground wires are only good for 20A circuits, max.

Putting a box in at the end of the future run, terminating the wires in nuts, and putting a cover on it is standard practice for a "future expansion" outlet of any sort -- putting a label on the cover indicating what it's intended for is a nice touch, but by no means needed by Code. In fact, you cannot "bury" the box behind the drywall -- it must remain accessible for later use/servicing.