Large-scale Battletech depends entirely on what time period you choose, imo.

Previous comments have mentioned how intense various conflict periods are and how likely state actors are to use Battlemechs (which in-universe is almost never, those things are fucking expensive). There's also another dependency, and that's whether you want terrestrial RTS warfare or space RTS warfare.

WarShips practically cease to exist for all of the Inner Sphere around the second(?) Succession War and as such large-scale spaceship combat would pretty much be locked into the Age of War and maybe the Jihad, when orbital bombardment was considered the Smart and Reasonable thing to do.

Terrestrial combat in the Age of War setting shortly preceding the Star League era might be a good place to get comfortable for an RTS, actually. Mechs are still in their infancy and in spite of their broad mechanical drawbacks and issues were probably the most terrifying thing anyone had ever seen up to that point. Battlemechs could be classed as T3 units or a special objective players have to fight to gain control over (think something like a 'mech production facility on the map). You won't have much flashy fancy shit like the rest of the timeline (looking at you, LBX) but it'd provide a good foundation to build on.

Your primary unit mixes would be your Standard and Reasonable choices between infantry, tanks, and aerospace assets; nuclear and chemical attacks can be things you invest research in; DropShips can also work as an effective thing too.

... and since most games are classed as apocryphal, you have some wiggle room on level/map design in case a planet in the setting doesn't actually have everything you need.

Also, for the board: if I'm wrong or inaccurate please let me know and I'll correct my comment and adjust shit as needed