Balance from campaign level will only make high tech strictly better than low tech
Isn't that exactly what "high tech" entails? Superiority? I don't have a problem with high tech ships performing better in battle than midline and low tech ships, but they should come with some sort of disadvantage - lower availability, higher costs, less reliability.
There's a few levers you can mess with in theory.
Option 1: Scarcity-
Keep high tech roughly better per ship, but make them harder to obtain. The difficulty with which you get most ships in the game, especially since blueprints, is roughly equal? Either you need a commission/blueprint, or you need some luck to find it to salvage/black market it. Some more luck than others, but ultimately its rng. There's a little bit of weight to it in that obviously you can go to specific areas to find what you're looking for, but beyond that it's the waiting game.
I'll admit i haven't had time to play too much since the most recent patch, but with relationships becoming a thing, having this be another avenue to better ships, while locking down other access might help limit the availability of high tech. I STILL think salvage should be looked at (massively dropped, ship prices upped, skills to increase salvage made more useful) but that's another story.
I will say i think this is risky. It's really really hard to gate content properly this way. The most memorable and relatable story to this would be Escape Velocity Nova. The polaris ships were, by design/plot, just better than everything else. Now in player hands it wasn't hard to build something game breakingly busted anyways(and several of the plot ships were also just bonkers), but polaris tech/toys were always the easiest way to do it. The game basically just shut 90% of that tech out unless you committed to that plot line...buuuuut you could usually capture the hull you wanted and at least start from there. Starsectors systems, as they are now, are not really setup for this.
Option 2: Maintenance-
You see this mostly with phase ships (having become high tech part 2?) but the idea that a high tech ship is great...if you can keep it working. Unfortunately this shows in CR, which is kind of an unfun way to deal with it. It works well for phase ships due to their hit and run nature, but having the entire high tech class limited by this is unlikely to be fun.
If new systems are added, the idea that you REQUIRE things like volitiles, rare ores, and what not, to keep a high tech fleet running could be a neat way to do it. As of right now trades goods are money with extra steps for the most part, so having more use for the various goods (also in building/restoring ships imo) would do wonders for the economy (another outlet besides cash conversion). Have the right pipeline to keep your fleet going? Great. Don't? Well then you probably don't want to load up on tempests or whatever.
This one is actually INVERSELY unfair to low tech right now, since they favor armor tanking. You'd think that the "held together with spit and rage" fleet would be cheaper to upkeep, but since they take real damage the supply cost adds up. I could see low tech being the "constant but low repair costs" but this really should be offset by high tech being the "you probably won't have any thanks to shields, but man when you get hit you're going to pay for it" side.
Option 3: Ship cost-
Another way to do this is to make high tech ships very expensive to replace in relation to low tech ones. This is kinda there, in theory, but in practice it doesn't do much. I can field more buffalo mk2's per dollar than medusa....but not that many more. Further they both restore/repair/salvage about as well post combat, despite one being SUPER hard to lose and the other being a poor mans space catapult. Comparing to slightly more similar ships doesn't do low tech any favors either, as the cost advantage drops off fast. There's not too much to say here as ship cost is mostly a function of its size, when i think tech level should play more of an effect than it does now.
I'll also add that "losing ships and buying replacements" isn't really a thing for most players anyways unless you choose to let it be. Both with how combat tends to work, and again how salvage does. And if anyone is losing ship it's low tech.
Option 4: More power to other tiers-
I'm not a huge fan of "nerf high tech". It's a valid playstyle and outside of things like the ion pulsar tempest drone, probably shouldn't be nerfed too hard. Instead I do think that the supposed advantages of low tech need to...well..exist. I LOVE the mora. Nothing is more epic, to me, than a combat carrier dead center in the middle of a melee holding on when it has NO right to, but it's one of the few ships in the game that I feel emphasizes the low tech doctrine perfectly (and is often my player ship when trying to stick to low tech only, after switching from a venture). I know there's changes that are coming in the pipeline to hopefully help this, but in general the "brawling" playstyle could use some love.
I also think "that's cute i've got 30 more" playstyle of cheap/expendible needs WAY more love. I'm talking a better ship system for keeping crew alive on suicide ships, and being able to deploy 15 buffalo mkII's over the course of a fight because that's how low tech could fight (i know that's really more of a trash ship from design pov, but I think the point stands ). Just throwing cheap ship after cheap ship at the problem until its solved. One of my favorite things to do early game is load up some condors with piranahas as one of the most cost effective ways to go station hunting, and again I think that's a really cool example of the low tech doctrine working super well, but stuff needs to support that more. Cheap, efficient weapons that while hard to setup, payoff insanely well (hell I'd kill for a low tech way to hold ships still for a pirhana run. Some sort of literal space harpoon pls? Gonna nab me a space whale?)
Option 5: Explicit Synergy-
This is something mods do more than vanilla, but skills or maybe hull mods that somehow reward you for sticking with low tech/high tech/midline. The game, as it stands, really doesn't give a damn how ragtag your fleet is. You can have a completely 100% doctrine following fleet, or you can have one of every single ship, doesn't matter. It would be nice if there was some bonus for staying in theme. Treat it like set bonuses from games like diablo. X deployment points spent on low tech gets you fleet bonus 1, Y gives you bonus 2...ditto for the other techs. Granted that's a whole new system so unlikely ,but something along those lines (just to encourage thinking about sticking with a style, not enforcing it) would be fun.
Overall these all have ups and downs, and the ideal answer is probably "all of them or at least parts of all of them", so we'll see where it goes. Ignoring my grand theories on the SC economy, I think just reworking supply costs for high tech/low tech ships, and making the repairs not nearly as brutal for low tech in comparison would do a lot (while making high tech FEEL more expensive would help). Breaking the games economy isn't hard, so players could still "do what they want", but it'd at least make the low tech players feel less punished for, right now, what feels like a sub optimal decision.