Didn't the Keas get their prospecting equipment (or whatever their main unique thing was) mostly ripped out to make room for the torpedoes in the refit, with that equipment already being obsolete?
Doing "the kea again, but better" is entirely acceptable when you remember that the current Kea is no longer actually capable of its original role, if that's the case.
The Kea actually removed the Dilithium prospecting module for extra antimatter tanks, presumably extending the range from 70ly to 140ly. If you consider the fact that the Kea was always intended to serve as a short-range internal explorer, then the loss of the dilithium module doesn't actually matter much since we've presumably already surveyed the vast majority of promising systems in our territory.
People actually seem to want a Sagarmatha replacement, which would make more sense as the Sagarmatha will be decommissioned by the time this ship is built. However the Sagarmathas are being replaced by the Excaliburs, and we know that Starfleet only ever operated 10 Sagarmathas and they have a greater number of Excaliburs already, so we'd really be banking on Excalibur casualties needing to be replaced.
Not mediocre, below average
…yes, that description of serviceable but unexceptional is what mediocre means. A C+ is not especially bad, it's just not good, either. Mediocre.
I want a multirole tactical cruiser thats competent at science; something better than the Excalibur at non-combat roles while being tankier and close to the same punch. Id be happy with a solid B in Science, and thrilled with an A
I don't think that's what Starfleet wants with this design. They want "a depth of capability in vital areas of interest", which suggests that we should be trying to specialize this ship using large modules that give it capabilities that smaller vessels simply cannot match. I don't think we'll get that if we try to sprinkle in a bit of everything.
And I disagree with your characterization of the roles of Science/Engineering/Tactical to Starfleet, and to the Federation in general as well; for one thing, your entire thesis doesnt account for the fact that the Federation has an entire sector of civilian engineering to handle non time-critical, non-military engineering and logistics duties
The civilian sector will not be dealing with phaser production or antimatter transport. They're not totally irrelevant to ship production, but they don't contribute to the things that bottleneck us. Presumably civilian production is why we can basically ignore the cost of mass beyond shields and thrust.
And that is essentially how the fields we're graded on work in practice, if you read the retrospectives. For example:
The Curiosity-class cruiser was United Earth's first dedicated survey ship, designed to investigate safe areas of space for potential colony sites and points of scientific interest
there are currently fifteen million colonists living on planets today that were first surveyed by the Kea-class
The Attenborough soon distinguished itself as a very competent survey vessel, identifying ideal colony sites and then landing to assess the immediate area in detail.
Science ships locate and evaluate potential colony sites within our borders, explorers go out and expand our borders by making first contact and cataloguing new stars, engineering ships respond to emergencies and provide Starfleet logistics, and tactical suppresses piracy and fights wars.
The only science that doesn't appear to increase Starfleet's obligations is strategic materials prospecting, but we've only ever been offered one module for that, and it makes way more sense to put that on a cheap ship like the Saladin rather than bringing all these other modules with you.
Anyways, my problem with putting more than basic science labs on this ship boils down to the lack of any roles that need to be filled. We already have internal science ships in the Keas and Attenboroughs and Atwater's, and we have explorers in the Excaliburs, so what else do we need? The order numbers for our last two designs suggest pretty strongly that Starfleet just isn't interested in more science ships.