^ This pretty much!
There are so many good historical ranges of models out there that can be used as KoM, its really a case of choosing what era/style you fancy and go from there.
For LotR style, obviously Easterlings & Haradrim work together - the former providing the knights, pikes and footguard (the sword/shield warriors), while Harad give you mounted scouts, bowmen, spears. It’s easier to find models that fit the Haradrim style better - Gripping Beast Arabs as a go-to, plus they have loads of spare heads which you can add to all sorts of other historic models so they fit the vibe.
Gondor & Ronan can do a similar job together. Gripping Beast dark age stuff can tie in with the latter, perry HYW stuff with Gondor.
Fireforge stuff is great - both the cavalry and infantry - and can cover crusader or generic western Europe, plus they do some sweet Russian, Scandinavian & Byzantine models.
Victrix have lovely viking/Saxon/Romanesque era stuff.
Once you have a core of men it is also really easy to diversify into Rhordia, Brothermark, OotGL, Varangur etc.
I’ve got a dark age style Varangur army, with lots of gripping beast, Victrix & warlord hail Caesar models, plus more fantasy esq units; an old GW empire one with warlord landsknecht & perry Agincourt models in quite a fancy, high fantasy vibe; another GW empire based one but with a grittier, HYW style, with Breton stuff, Fireforge etc; an Eastern/Arab style one with lots if gripping beast models, camel riders and pretty much anything I can stick a turban on!