I guess it is more of a preference. I feel 10kt ships are still small ships and need specialization to be effective. In terms of ASM ships, I prefer large salvo size than the multi-role capability on ships smaller than 20kt. As for the large sensor, it is more for convenience rather than creating a tactical advantage. Although obvious in a fleet, its cloaking device should make it hard to be targeted by long-range missile fire. Each destroyer is fitted with a suitable active sensor for their own role, so in the worst case of the sensor ship is lost, the fleet keeps its fighting capability.
The heavy cruisers are more of an experimental to me as I don't build large military ships much in previous games. The designs are to show the effectiveness of the larger ships after the buffs for them implemented in C#. They are slower to build, but indeed more cost-effective compared to smaller destroyers. The destroyers still exist due to their flexibility to reinforce any fleet and their lower cost and build time, and the fact that I do have quite a few shipyards with many slipways dedicated to mass-produce them.
Something a human can do to target the sensor this is to simply use EM sensor on the missiles, so you fire the missile against something and when that thing is destroyed it will re-target into the strongest emission which is the sensor ship. Or even sneakier would be to turn of the targeting senor just before impact as the missile the re-target into the sensor ship.
But the AI obviously never would do any of that so why not... my idea of sensor scout is that it is much cheaper and way more flexible and make it actually possible to use to actively scan for enemy without revealing something critical as you entire fleets location.
But I look at this from a multi-faction earth start where you have needs to be efficient all the time and rarely have time to build anything truly unified. You need the ship yesterday not tomorrow kind of thing. So it is different for different conditions.
As for shipyards I can understand that you would like to continue using the yards that you have, I would still contemplate expanding them or find some other use for them and make all my capital ships larger. I have personally never seen any campaign where I could not overproduce ships in any category no matter what. There really are no way to replace those particular destroyers for a cruiser as you a NEVER deploying less than three destroyers anyplace anyway, so they are not cheaper in any way.
The only time I would feel a need to produce ships very fast would be in a defensive situation, then I would use my 1000t shipyards that each have ten slipways before anything else to produce things really fast... otherwise these shipyards are for refitting small crafts.
There is nothing wrong with using many smaller ships, I just would never do it if I have the components to build larger more potent ships. I would just expand my current yards to take full advantage of it and that is what I normally do. For me a 10.000t ship is a small ship... a capital ship that can't deploy on their own in some fashion is not really a useful ship. I always have to risk a large number of crafts for no good reason. If I want a powerful force I want it to be as survivable as possible, that means as large as my research can allow me to make them. Expanding the yards and slipways are generally much easier than doing the research... now with 20% research rate it is super easy to do that which changes the perspective allot to of course.
In the end... the size of ships in my campaigns depend almost entirely on my research capabilities and the outside pressure for an offensive fleet... defensive fleets are quite useful to build just very small highly specialised ships as they don't need support of a fleet structure or the deployment length to do what they need to do. Defensive ships I rarely make much larger than a few thousand tons, maybe as large as 5-6kt or so but even rarely that, FAC and up to 3-4kt max usually.
In my current Conventional game the first real capital ships I look at making right now would be around 10kt and that is with Thermal Nuclear Engines technology, so very early indeed. But I have no need for offensive military ships as I have just started to explore at this time. In another 10-20 years I will have the yards and components to likely support capital ships at 15-20kt most likely so they will be full fledged multi-purpose at that size.
I also would never field military ships without beam weapons... beam weapons is what infantry is for ground wars. Beam weapons make you hold key areas in space that does not move. Those are the areas that are why you build ships in the first place. That is colonies and jump-points. If you don't have enough beam weapons you will have to fall back after you launched all the missiles and in beam combat have some beam weapons on every ship makes you way more resilient and likely to win.
You then also have the issue of command staff, larger ships can get the best and most efficient command staff.. getting enough good captains for huge numbers of ships is quite difficult. This issue should also not be taken lightly and is a serious consideration everyone should make. It is good to weed out the good and less good captains by staffing the less experienced ones in your more defensive backwater defence fleet and the best one on your larger offensive capital ships.
I also know and fully acknowledge that there is not really any right or wrong in however you do it... ultimately I don't think it will matter all that much to be honest. The above is just my general thought process and why I tend to do something in a particular way... I tend to allow my fleets to evolve more organically and never really settle on any particular size or speed or whatever and let external pressure decide that for me.
A final note on a really long reply... I would try to find a good different use for my destroyers. I would make them perhaps slightly larger and mutli-purpose and turn them into forward scouts I can deploy with cruiser battle groups. Perhaps much better speed as extra defence. I could then deploy them as a screening force... say I have three or four cruisers and three to four destroyers... the destroyers can still add some punch to the battle group but also act alone or in small group and using their extra speed to scout and evade enemies. I just would make them clearly different with a different purpose... I would not get rid of them.
