Corvettes are smaller than frigates
Make t3 navy more exciting!?!
Corvettes are smaller than frigates
Yes, though wikipedia says "slightly smaller" 2000 tons used to be considered the limit, but a 3000 ton class is claiming Corvette title now too, vs frigates that are 2500-7200 tons.
I don't know how often you use Aeon Shards, but their behavior and maneuverability is basically what I suggest - the size of the Corvette should be larger than the Shard, smaller than Frigates, but ultimately that decision should be dictated by being properly recognizable.
Obviously, they would be very "power dense" carrying advanced looking weapons to be credibly better than frigates. The Cybran could for example launch "Iridium rockets" similar to the Hoplite Rocket Bot. That weapon is smaller than the "Proton Cannon" used by regular ships, but it is visually credible that the "Iridium Rockets" are better and deal more effective dps. Personally I would make them homing for the Corvette and reduce the fire rate to tune the effective DPS to fit the "Advanced Weapons" look with the desired DPS.
For Aeon I think the Obilivion Cannon carried by the Specter gunship looks right, for UEF the Heavy Plasma Cannon of the Titan looks right. Seraphim as mentioned a Shie-eih Auto-Cannon like the Frigate and a Thuntho Artillery Cannon like the Zthuee because Seraphim are weird and don't follow the rules.
Secondary weapon loadout should be determined but I don't expect the balance to require them to have significant or any anti-sub or anti-air weaponry.
I think that battleships primary purpose is less about fighting other battleships, an more about shelling shorelines, and inland bases. in that role, it's actually fantastically balanced. battleships are a extremely durable, consistently damaging, mobile artillery pieces. essentially more comparable to a t4 than a t3. changing them to being glass cannons would make them extremely swingy.... easy to sink, a massive resource investment, and the consistent damage to shields it used to do would turn into utterly overwhelming on shore firepower. I think that if your building mass battleships than something is going wrong, as that's really closer to mass t4 spam imo. t2 navy really should be your primary focus in the late game, with maybe a 1-3 battleships. if the sim speed slowing down is an issue, you might want to remember that your playing on setons. sim speed is always going to be rough on a map like setons.
Tech level is about cost, a tech 3 Corvette that costs more than destroyers and frigates doesn't make sense especially when there isn't a t1 corvette. I think if we to add a unit a t3 frigate with all around better stats would be a better addition.
There isn't any push from the devs right now for adding units, so I would try other fixes right now than unit adding especially a unit addition would totally break navy and would have to rebalance everything.
Maybe add more range less dmg and less hp to frigates
Without being on the balance team I can already tell you that there won't be new units. Maybe even ever. It is incredibly hard to balance a new unit, because it is impossible to forsee the role it's going to end up in (at least on a higher level). It will most definetly not turn out as you planned it to be and you have to constantly adjust the stats, to keep up with the meta changes.
now that I have time I might fiddle around a bit with the BS acceleration and speed, could be quite fun to play with low acceleration, so you would have to plan in momentum.