Hmm... so you're basically saying that early melee is too powerful compared to later variants, and it scales too much with stats? (one-hitting powerful enemies in melee when your stats are high is, I think, quite all right, not really breaking the system - there is also the problem - if melee is too weak, you start to think about how risky it sometimes is - even though enemies don't react to melee attacks nowadays, which is a major advantage). Hmm... cutlass peaks at about 95 damage, while the Electro-Sword at about 135. While there is difference, it is indeed not that pronounced, especially since 95 is a LOT anyway...
Indeed, the difference in peak damage is small and you "quickly" get to areas where you don't really need more. Instead of 40 more damage giving you a one hit kill, you can often spare the TUs and stamina to strike twice and get the same result. But the main interesting thing to me is that it scales differently. I don't know how much damage the endgame melee (or ranged) weapons do, but it is undeniable that there is a combination of factors that favours melee regardless of weapon tier in how they advance and actually keeps entry level weapons (axe, hammer, cutlass, billhook) competitive rather late in the game in the hands of experienced gals, while muskets and rifles have long been obsolete regardless of who wields them (actually, can still be good as cheap training weapons for taking potshots for experience.. but I don't do "training exploits").
Quick scaling:
Ranged: More TUs doesn't shoot more. More accuracy hits more, until you reach ~90% (which is fast on a few weapons). Reactions + accuracy helps for snapshots on enemy turns, but that's really risky (snap accuracies aren't great, ranged damage isn't either, and the enemy can get mutual surprise or just flank you, so it's risky even in end game). More stam is pretty meaningless since you spend most of your TUs shooting and don't need to move that much. Str is maybe your biggest boon because you can carry a bigger gun. That's pretty much your only way to improve damage, since it rarely scales otherwise.
Melee: More TUs mean more strikes. More accuracy hits more and it takes longer to reach 100% to hit. More reactions increases your survivability by preventing reactions against you (it's risky to charge, but it gets less and less risky), more stam your mobility. More strength means more damage.
So there's only 2 ways to improve ranged damage: Better accuracy up to a certain, sometimes easily reachable point, and a bigger gun (big difference, but it means lots of research). There's 4 ways to increase melee damage: Better accuracy, better TUs, better Str and better weapon (but as you said, not that much of a difference).
I like the way things scale, the fixed TU cost of melee is nice to represent faster strikes from faster gals and the increase in damage from str is obviously needed. I think this works well in the early games, when gals are mid-ranged stats-wise and have rather low str. But once you get experienced gals, the melee ones benefit a lot more from stats gain (pretty much everything is helping) and can do things with starting melee weapons that are really impressive. This, in turns, makes higher end melee weapons less exciting because you are already "fine", whereas upgrading guns is primordial since it is close to the only way to improve ranged output. Which is a bit unfortunate in two ways: It's much harder to build a ranged monster, and it's much harder to be really excited about a new melee weapon because you already have a monster.
I'd like to see some scaling in damage on more ranged weapons, rifle +5-10% of accuracy? Snipers +10-15? Or maybe scale with reactions to encourage people to "risk it"? Reactions then also becomes good for shooty gals as they react to targets can spot weaknesses faster. That doesn't change much but it adds an extra scaling to ranged combat to restore the balance a bit. Or have some more ranged weapons with fixed firing costs, so increased TUs on the gals mean they can fire (or run) more, like it does for melee. If an experienced melee gal can strike "faster" (TU cost is a smaller % of her TUs), why can't an experience shooty gal aim faster? She won't hit for more damage, after all. Or if she takes the same time to aim than a starting gal (same % of TUs), then she should hit somewhere more painful since she is a better shot (back to damage scale with accuracy).
Melee also has another advantage: 12 extra TUs and you went from front armor to back armor and 12 TUs is a small fraction of your total. For a shooty gal it's often impossible to both flank and shoot, but that's somewhat mitigated by being able to spread out more so some gals are already flanking.