Ah, great memories.
I played all DLCs and the original game. They are all the same, but in different settings and all are awesome.
Yes, you'll have a lot of backtracking after boss fights, but it didn't bother me. Just invest everything you have in leadership and you are good to go.
HoM&M III - my respect, mate. Among 5 best games for me.
Not just boss fights, but any fight that involves losing units since I like to keep unit stacks at or almost full strength.
I think that would be a bad idea dumping everything into leadership. Sure, you'll have a lot more units per stack, but they're going to be weaker; especially if you're playing a mage, which is what I'm currently playing for my first time through. For this particular character, I put most into intel, followed by defense, and maybe a couple of points into attack. Right now I have around 11 attack / 24 defense / 32 intelligence / 116 mana / 69 rage and around 18,000 leadership at level 27.
Another problem I foresee with going for really high leadership and not putting enough into attack & defense is you won't be able to replenish the units enough to keep them at full strength and you'll eventually have to replace the low-count stack with a different unit from somewhere else that hopefully has sufficient numbers to recruit. Orc Veterans are probably one of the best humanoid melee units, but unfortunately, I was only able to recruit all of them once at the orc stronghold/castle in the Western Islands. Haven't found them anywhere else yet.
This where KB differs quite a lot from Heroes. You really need to take care of your units. You need to be mindful of synergies between them and not recruit something just because you can. I remember I have finished one campaign (can't recall which one exactly, I think it was the second in the series) without losing a single unit. Some units died in battle, of course, but I had everyone resurrected by the time the battle was over.
Tbh, I found KB frustrating at first. But after I learned its ways and stopped playing it like Heroes, it became really enjoyable. And the difficulty was part of the fun.
Resurrection helps, but its not always an option as that spell eats through mana quicker than most other spells. By the time I need to start using it, I'm about 2/3 through all of my mana from blasting away troublesome units early(i.e. necromancers, enemy archers, stupid wolves with fear effect, etc.) and sometimes end up having to slow the last enemy stack(assuming its under lvl 5) and drag the fight out until I regenerate enough mana to cast it on all the units that need it.
I do take care of my units as I figured out very early in the game that units don't restock often enough to zerg your way through all of the fights. So far I've only had two instances of running out of units: Archmages(which I was able to use again when they replenished at their recruitment locations), and Veteran Orcs(which I fear won't be returning anywhere). Its just a pain in the ass having to run to different locations to build up or rebuild the army.
Right now my army consists of Black, Green, and Red Dragons, Archmages, Shamans, and Priests. I try to nuke as much as possible in the first 2-3 turns since I can use spells 2x per turn 3 times. With my current intel stat, my highest damage spell is Ice Snake which does 4200 dmg at lvl 3. I'm finding in the Demon area some of their units are a pain, such as the Arch Demons with their really high defense. Those Cerberus dogs were a real problem for me when I first encountered them on the dwarven continent in the mines, too. Its a good thing I have good defense or I would be losing a lot of units in the demon area with their defense bonus. I'll have to experiment with some more different units.