Hm... It really depends on what they're fighting against, honestly; most of their default variants were heavy on HE weaponry, which made them overall pretty non-threatening to my preferred tactic of using very shield-tank-y ships - the only thule fleets I really made sure I avoided were those with lots of fighters, since the fighters all had decent kinetic guns, and a swarm of fighters + gunships could easily ruin your day.
Now, one thing I could see as a possible faction theme would be going heavy on the kinetic damage - taking the thought process of "If we don't have shields, you don't get to use shields effectively either." - in this case, swapping the Barbarossa Cannon & Heavy Slugger (& maybe that one extreme-range medium gun I forget the name of) to energy damage would make sense, and then use a mix of those & HVPCs to have loadouts that rips through shields, but are merely okay against armor.
Generally, though, I think this falls into the same issue that the Gedune's Tenzen Cruiser had: You're trying to balance your ships by adjusting the weapons - which only works for the AI; if you weaken Thule-specific guns, the player will just buy classic weapons; if you strengthen the Thule-specific guns, the player will use those on classic hulls. Have to balance the two components individually.
As for lengthening combat with Thule ships... I might try things like doubling their hull hit points (or making a thule specific hull mod that reduces hull damage taken?) and then reducing overall firepower as needed (fewer slots, or slots that don't overlap as much, or even just reduce flux vent rate). ...On the other hand, that'd make it even less likely for me, as a player, to want to use Thule hulls; if I expect them to take hull damage in an even fight, I'm going to install a front shield generator, or just not use them at all.
