So, what kind of military would an ambitious, prosperous independent world with an eye on expansion muster?
I’m thinking it’s obvious to lean toward a lot of infantry and armour with limited Mechs.
But how many Mechs? Is a battalion too much for such a place?
And what about Mercs?
In the canon DA universe (which I don't fully subscribe to, but eh) many minor battles are being fought with LCTs made up of a reinforced battalion of Mechs and essentially a battalion combat team, as opposed to regimental.
Hence a militia should experience the same 1/3rd downsizing. A strong 3067-era militia would have at most a battalion of Mechs and full supporting arms; a militia in 3130s with a third of that should be very decent.
I mean look at the DA books, the Swordsworn are basically the size of a planetary or (small) regional militia and they're shuffling company-sized forces around. Aim for that.
Composition - for inspiration, take a look at say a NATO brigade, and think of their BT equivalents. Off the top of my head, you'll need Mechs, tanks, infantry, BA (aplenty in DA era), artillery, ASFs... Support arms like engineers for fixed defences, minelayers or chemical troops for sneaky smoke shenanigans, medical because you need to conserve forces, etc...
P.S. re your new post - a less moral Govt wouldn't mind doubling down on the Infernos and flamers and whatever nasty stuff you can think of without going full WMD. The suggested force size is too large I think, see above. And 3060s equipment is not a problem cause post Jihad this stuff is accessible to miltiias too.