Space on a flat deck is limited. I agree that an aircraft with heavier payloads may be more ideal in such a scenario, but there is a trade-off.
As for the 076 being a straight deck CATOBAR, keep in mind that these fixed wing aircraft are unmanned. They are smaller in size and simpler to operate. Moving aircraft from the forward part of the ship to the aft part or to the launch position also should not interfere with launch operations (edit: though this will need to be confirmed for certainty), which is not the case for other LHAs/LHDs. So while ops are obviously limited, they're also likely more efficient in comparison.
No disagreements otherwise
Well, I agree with you in the sense that GJ-21 on 076 will still offer a useful strike capability.
But the geometry of the flight deck IMO ultimately is still going to be tough for high optempo sorties. The smaller size of GJ-21 relative to a standard manned aircraft is also countered by the smaller size of the 076 flight deck compared to a proper CATOBAR carrier (even one like CdG).
The good news is that they don't necessarily have to move aircraft from the bow part of the ship to the rear during a launch cycle simultaneously as they are launching from the catapult. More likely that in a launch cycle they will prespot that cycle's aircraft in the aft half of the ship, while the bow spotting positions accommodate the aircraft from the immediately preceding recovery cycle.
If anything I would say the value in 076 being a CATOBAR with GJ-21s is the ability to conduct organic relatively high end air to ground or even air to sea strike missions at long range and in contested environments.