I'm looking for games that feature certain elements and I'm unsure what to call this particular type of game that I am interested in, so I'll settle for "mercenary sim" or perhaps "warlord sim". Basically I'm looking for games that are more or less like the Mount & Blade series. Games where the player makes a character, adventures around some kind of open world (even if that world is mostly a map as in MW), acrues a following of soldiers or helpers, levels up, perhaps gets to own a base, quest, and battle parties of enemies. Anyone know of games that fit this particular niche?

Games that seem to fit this label that I'm aware of only include:
Mount & Blade series
Sid Meier's Pirates series
Sword of the Samurai
Kenshi (?)

Freeman: Guerilla Warfare is supposed to be "like Mount & Blade with guns" - though it does sadly appear to have hit development issues recently.

https://store.steampowered.com/app/773951/Freeman_Guerrilla_Warfare/

7,62 High Calibre may also fit (its kind of a "Eurojank" Jagged Alliance but real time with pause). I Got a lot of hours with it.

https://store.steampowered.com/app/289890/762_High_Calibre/

Cataclysm DDA is a roguelike where you can get people to follow you, build up a base etc.

https://cataclysmdda.org/

Rimworld also fits to some degree i suppose.

https://www.gog.com/game/rimworld


Ostranauts may also fit to some degree (at least once more development is done) or be of interest.

https://store.steampowered.com/app/1022980/Ostranauts/

I think the squad or troop management and strategy elements are essential for what I'm looking for. I think it's one major element that separates what I'm looking for from many open world RPG's.

Overlord I and Overlord II fit most of the description in the OP.

Wartales

Don't think it's released yet

trailer: https://www.youtube.com/watch?v=68L5J9bDYsg

EDIT: I didn't really read the first paragraph hehe, my bad. Then I take it you won't be interested in the kind of mercenary sims I've listed below. But I don't plan on deleting them either, since I think it's also a good idea for you or anyone to give these games a look.

Mechwarrior 4 Mercenaries is also another mercenary sims you could try your hands on. From what I know, Microsoft had declared the game to be free to download a couple of years back.

Battletech is also a good mercenary sims in my opinion. That said, the game is really unoptimized and many complained about the memory leaks that the game has. If you're not bothered with that, then feel free. Take note: I don't think it's necessary for you to also purchase all the DLCs for the game (because you know, Paradox), since the community has come up with mods that not only expands the world map (the galaxy) that you as the player can roam around in, these mods have also added a bunch of new mechs previously absent from the base game (and I think also from the DLCs). Kenshi is also a good game. But, I think the game falls short when it comes to the amount of quests that it offers to the player. You just collect bounties from defeating certain people, earning money from selling ores, etc. The faction leaders however, do provide unique quests to the player when you meet them (at least that's what people say, since I've yet to encounter them so far (surviving is already hard enough lol)). So compared to games like the Mount & Blade series, Kenshi ranks below it.

That one looks interesting. Thanks! I added it to my wishlist.

Battle Brothers was also going to be my recommendation as soon as I saw the thread title, so I'm glad it got recommended early. The game is pretty much the definition of "mercenary sim", and combines excellent tactical engagements with the strategic element of building up and gearing your mercenary company, plus the management aspect of which contracts to take and how to manage your company's resources and finances. Be advised that the game can be brutally hard, with death always right around the corner for all the brothers you've spent time lovingly building up. The only piece of advice I'll give is that risk management is a critical element that many new players overlook- you always have the option to give up on contracts or withdraw from battles, and sometimes the best option is to pull a Sir Robin and beat a very brave retreat.

https://www.gog.com/game/port_royale

There's whole series but the first one was probably the best.

If you don't mind hard difficulty and facing enemy fleet with one of your ships at the time, then:

https://www.gog.com/game/port_royale_2