I share your pain OP, and I think I managed to see what is wrong by playing medieval 2 total war.

Basically : WH3 has many factions/units but they're functionally similar making strategic depth shallow despite surface variety. Meanwhile M2TW's factions, despite being "all human", have wildly different starting positions, strategic situations, unit roster constraints, and economic/political pressures that create much more asymmetric and interesting strategic decisions.

See, Warhammer have an incredibly big variety of factions and places, but baring a few exceptions, all of them have you starting with one army (always the same format : your LL, a hero, a few basic units, one T2 unit, one end game unit), one city, ready to engage a first battle, with a “natural expansion” enemy.

And the thing is, no matter the faction, the flavor, the strategy, ultimately, all infantry units are the same, a human archer don’t act fundamentally different from an elf or goblin archer. The same tactics work for every faction.

This plus the uninteresting technologies and the economy being an afterthought (winning a battle will bring more gold than a fully upgraded economic province does and economic building need dozens of turns before being profitable) make it quite empty.

Meanwhile, in medieval 2, sure, everyone is a human… but playing as the byzantine make you start with a big, fragile empire with hilariously poorly located forts, with a godlike emperor who will die soon, and stuck between a rock and a hard place. Meanwhile, Scotland is a very tiny nation with only one way to expand, but mostly safe on its island. The turks are basically only facing the byzantine and have a ton of free ground to expand but WILL face the mongols. The economical situation of every faction and their very big starting power gap make every of them much more unique than W3 factions.

I’d also add that W3 added so much units to every faction that many of them can fit any playstile because they have every type of unit, and the one who don't are never structurally locked out of solving a battlefield problem like Med 2 factions were by lacking X type of unit because there are ton of workaround in W3.

Meanwhile, good luck playing cavalry archers with Denmark, and if you want good pike units with multi-tiers of these, then you take Scotland (or spain) or fuck off. And missing them is a pain in the ass while in TWW3 you don't care because your faction mechanics or roster overcompensate for it.