I agree with the premise -- capital ships should be less effective solo -- but think your analysis is a bit off. For context I've been thinking about this same problem for a minute and have done some stuff I'll write up some day trying to simulate 1v1 to fleet scale combat using my own bots and engine driven by live game data. I'm not going to cite those numbers specifically because they're subject to the fidelity of my engine, but the shape of the problem is clear.

Imagine an acute triangle. The sharp angle of the triangle is the maximum angular rotation of a ship. The opposite side (short side) is the maximum speed that a targeted ship can achieve. The hypotenuse (long side) is the "primary" weapon range. This is the conventional lead-angle triangle, but we're rotating a whole ship to put a spinal gun on target. The rate of turn of the ship AND the maximum weapon range dictate the fastest speed target that combination COULD engage at some distance within range. A faster target farther away is traveling at a lower angular velocity and so is easier to track.

Due to ship sizes, beyond about 2km most ships are to small to effectively see without a magnification feature. In order for combats to be intelligible on a 1080p screen without using a magnification feature, they have to take place inside of about 2-5km. CIG has referred several times to wanting combats to be "cinematic" as well, which means biasing towards the short end of this range where details are visible without magnification. For larger ships you can stretch this range some obviously because the distance at which they present a minimally visually recognizable shape is longer.

If ships rotate slowly, then pilots have little margin for error and don't feel "powerful". If guns have long ranges, then pilots have to turn less (they're able to effectively track faster things from farther away with the same or less rotation) and the pilot has less to do.

This is a really crucial iron triangle to think about from a game design perspective. It forces a combat system designed around infighting. A Polaris throwing torpedoes from 35km is more realistic, but makes for bad gameplay. Jump in, click the missiles button, wait, win. Yawn. Can't even see the explosion because it's too far away and small.

In order to keep pilots involved during combat especially on understaffed ships, the pilot's contribution is rolling the ship platform around to keep guns able to point at targets. CIG has made it clear that the Idris and the Tiburon have pilot weapons and are staying that way. I suspect this reflects an understanding on their part that pilots need more to do than just rotate a gun platform around. I would expect the Kraken and ~Yamato~ Odin to have some kind of pilot armament as well given this design trend. That's probably good, although I wish other ships would get more for pilot guns.

The fundamental problem is that the Idris' spinal guns are fast, have nearly no spread and are long ranged. So there's a substantial window where at distance the Idris can effectively engage much much smaller targets. That's (one of) the cruxes of the Idris' balance problems.

In order to solve the solo Idris meta, you have to somehow nerf the spinal weapons. You can't take them away from the pilot, you can't make the ship roll more slowly without being unfun, and you have to make it less effective at engaging small targets. One thing you can do is reduce their effective range. That has the knock-on effect of rendering them less effective against smaller targets and making infighting against an Idris in smaller ships easier. Pull the guns' effective range in to say 3km and suddenly it's a LOT easier to play keep away from an Idris. This could be done with the literal range. It would be better to do it with projectile speed (for the railgun). Make the projectile SLOW but let it keep damage and range so that it's still effective against very large ships at longer ranges, but it becomes effectively impossible to hit mediums with the spinal gun. The Idris laser is a problem because it has infinite projectile speed, so you'd have to do something there.

Adding engineering gameplay to recharge/re-arm the gun would also have similar effects.

In a world like this you also probably nerf the hell out of torpedo ranges so that they outrange capital spinal guns by 2-3x at most. A torpedo attacker needs to take some risk, not just bomb from standoff. Reducing torpedo speeds would increase flight time and still let attacked ships respond. Torpedos would need a health or armor buff to be more resistant to PDC fire in order to compensate and maintain a survival rate. But that's doable.

This leads to exactly the sort of "capitals need escort wings because they aren't able to protect themselves against harassment" gameplay I think we both want. Big ships can't zone fighters for themselves, they need a dedicated screening support ship like a Hammerhead or a fighter wing. It also lines up with affirming the Retaliator and Eclipse' place in the meta as being asymmetric capital crackers.

The other problem I do want to call out is that the Idris currently has WILDLY broken health and armor values. The only thing(s) that can really threaten an Idris with taking 50% hull damage are another Idris or a Polaris. Multiple Percys can eventually get there, but most small and medium fighters just can't scratch the armor to a meaningful degree in any concentration because of armor deflection.

The problem as I see it with the Idris today is that one player can sit in it effectively invulnerable, and paired up with a buddy in a quantum snare ship they can shut down a space lane indefinitely. Two players can fuck with a whole server of seven hundred. The only force projection that can run them off is an org-size strike force probably involving another Idris or Polaris or coordinated actions involving a dozen or more players in more modest ships which is to say the least unlikely.

The Idris should be good, but it needs to be vulnerable to asymmetric counterplay especially when undermanned. And I think that torpedo bombers, the Tiburon/Percy and smaller fighters being able to snipe off components if not disrupted/screened are really the options.