The “no shields” option is definitely… intriguing. From a doctrinal standpoint it definitely changes a lot of how more advanced fleets would want to be designed.
One, with how missiles now work you’re now unable to simply absorb the damage from leakers with shields. Now that amounts to “chip damage” that’ll accumulate and ultimately overwhelm your ships if their armor lacks sufficient layers to keep up for the whole span of combat. Two, particle lances become exceptionally more terrifying because armor is now your only real line of defense and particle lances care not for your armor.
The main outcome I see that this leads to is that any ship combat that falls within the range of particle lances becomes a “high risk, high reward” scenario where either fleet would mutually obliterate each other until one or the other comes out on top. Which is to say, particle lance versus particle lance would lead to high-casualty combat for both sides.
The only deterrence to that is range, which effectively constrains combat to having capital ships at long distance to each other. Long-distance lasers would push this distance out even further. The actual combat would happen either by way of missiles or fighters. I can even see the actual combat happening somewhere between both capital ships, as fleets of fighters are sent off to act as a stand-off point defense against incoming missile salvoes, leading to anti-fighter-fighters being deployed to destroy the enemy point defense screen.
I suspect that plasma carronades would fall off heavily in utility, since getting a ship equipped with such a weapon in-range to use them would be quite a challenge and the resulting output rendered fairly ineffective by way of the sheer number of layers of armor that opposing ships would normalize having.
High-powered microwaves would also have high utility, at least early on. The issue becomes that you’d eventually just invest more in hardening, which doesn’t up the cost of the component in 3.0. The only countermeasure aside from “more microwaves” is “larger microwaves”, and the problem there is that to mount a larger high-powered microwave you would need a larger ship. That larger ship itself becomes a larger target, and given all the other factors in play forcing a heavy emphasis on combat-at-a-distance, such a ship by the time it actually gets into range to attack the enemy would be vulnerable to significant counter-fire from long-range lasers and opposing high-powered microwaves. Plus an enemy fleet could just use their own long-range ships of these sorts at standoff distance to continue to defend their capital ships from direct attacks.
In short, I would expect without shields that combat would become a very long-distance affair, with multiple concentric layers of ranged offense acting also as stand-off defense. The outer-most layer would be missiles, followed by fighters acting as long-range point defense and as anti-fighter efforts to break through the enemy’s long-range point defense, followed by long-range lasers and high-powered microwaves to keep enemy long-range lasers and high-powered microwaves from moving into range of the capital ship, followed by some assortment of particle lance ships to effectively annihilate any ship that comes too close, followed by the capital ships themselves (carriers, missile ships, and sensor ships).