The latest upgrade is good. But what I want is more space to build. Not larger maps, but more flat space.

Try the Flatter Map mod found here - https://www.nexusmods.com/dawnofman/mods/

Not keen on using third party mod managers. My gripe is that current maps are large but with very little usable space.

There's no need to use the mod manager of Nexus, you can just download the map and add it to the game.

Edit: whoops turns out that in this case you do need to do that. My bad.

You don't need the mod manager for custom scenarios at Nexus, such as Jericho and Exile, but it is needed for all mods.

With the limited game play capabilities of DoM, I'm thinking the small amount of buildable flat land is intentional.

The highest population count I've read about is 254 and the possibility of building a satellite settlement is minimal at best, since the people still wander back and forth between the 2 settlements when they could do whatever they need to do at one, if they're built right.
It doesn't matter if both settlements are built identical or not, they still roam back and forth between them to do their 'duties'.

Because of these current game limits, I honestly don't see much need for more flat land.

Don't get me wrong. I'd love to be proven wrong on the points I made.