So, it seems like what's missing from the rules as they stand is some form of conversion of total BR into a smaller attack-modifier value, one that can only go up into the low double-digits so that when you add a d20 roll you can get a significant variance in outcome. I'm thinking something like +1 per 50 BR, but that's probably too simplistic, and it feels like there needs to be a diminishing-returns element to it.
To further simplify the in-play maths, you could give units a Defense bonus equal to that modifier +10, to eliminate opposed rolls.
Aside from the maths issue, I like the general concept of the rules, but I'd like them to take more account of leadership, especially from particularly powerful or cunning commanders. I'd like to see some form of "Commander Actions" concept - not something they can do out-of-turn like Legendary Actions, but ways that they can modify the available actions of their units.
For instance, a horde of orcish barbarians under a ferocious berserker leader might have the Relentless Charge action - when they use the Attack action after moving adjacent to an enemy, they can push the enemy unit back one square.
Or a group of kobolds with a particularly clever warchief could have the Cunning Traps action - when they take a Disengage action, they can pelt the enemy with tanglefoot bombs and scatter caltrops in their wake, leaving the opposing unit pinned in place for one round.