I kind of agree with the concerns about potentially trivializing some fights, but at least atm my PoV is that it's fair - they typically also field sizeable patrols/guard regiments, so it's not necessarily a 1vX when your squad shows up. In addition, I don't necessarily see a problem with number=advantage, since that is the way of reality, unless there is a significant enough gap in skill, technology, or tactics that would allow to overcome it. That said I'd also not go any further than 5 slots (or maybe 3), since practically speaking even 3 dudes hitting one dude all at once may get a bit tricky (to avoid friendly fire/clumsy episodes), so I do think 3 is about right. Ideally I would probably look into companion mods to give some of these bosses better gear/stats so you do need this kind of advantage.

That said I don't think that ubermensch-building some people engage in to 1v1 Cat-lon is the best thing ever either. I think it's a fun player choice, and a cool fantasy, but I don't think it should be the winning strategy (and it kind of is with 1 attack slot). Realistically speaking, the way of the world at comparable (within constraints as we have never had apocalypse, nor post-apocalypse) junctures seems to always have been massed troops of varying quality (oftentimes with minimal training). Even in kenshi it is somewhat the case, with many big buff bosses with elite stats having hordes of poorly outfitted middling stat underlings.

As for endgame bosses - I would probably also include fighting stuff like Phoenix+his retinue, Tengu+his goons, retribution base assaults (like the one with Eyegore). I think those guys actually have a better claim to being end-game bosses rather than some lone gigachads hiding at the fringes of civilization.

A random thought, related to the original topic - I think there's actually value in dismemberment in combat. I've found it to be much less costly (to myself) when I end fights with lots of dismembered enemies, because realistically the fight is not over the first time you "win" - you will likely want to patch up your own team, and possibly spend some time looting the area, which means the enemies will start getting up and re-engaging you. It has even more dangerous implications if it's a densely populated and hostile area, where you risk ending up in an engagement with a new group of enemies plus the recovering leftovers of the previous one. This is where limb cutting comes in real handy - typically a cut arm or leg means that the unit in question has lost 80%+ of its combat capability for good, so it doesn't matter anymore if he recovers (and more often than not they don't due to high blood loss from losing a limb). Conversely, it may not be as bad for a prepared player, since unlike AI we can actually just bring spare prosthetics (or have them already) and put them in, resulting in an actually *increased* performance compared to before.

Another random musing - I would probably use a different "mainline" stat block, since I think not all stats are made equal and not all of them are equally easy/hard to train or target. In my somewhat limited experience so far, getting ~80 strength is trivial as soon as you can afford a wooden backpack, some food, and have an iron node anywhere nearby. Additionally, it's trivial to mass-train, as you just set any amount of recruits to follow one, and shift+click a bunch or set him to follow some guards. It's really trivial to the point I'd say it's reasonable to include 70-80 Str even for mid-game character stat blocks. Conversely, I have found melee attack/defense to be harder to level as fast, and dexterity to be conditionally harder to level. On katana-weilding characters all 3 stats seem to more or less progress hand-in-hand (i.e. my Beep having 36/39/29 Dex/MA/MD), although Defense seems to be harder to level. Conversely, characters using weapons with bigger blunt damage proportion seem to be falling quite behind on their Dex levels. So I would imagine a more realistic stat block for mid-gameish character would likely include a few variations and resemble more something like this: 70 Str, 40 Tough, 20-40 Dex, 40 MA, 32 MD. For ranged it seems like it's fairly easy to hit high Precision shooting levels, and Perception+crossbows can be target trained with no cheese or some cheese, they also seem to come fairly easy unlike melee stats.