You're still pushing my good troops - micronukes, battledress, anti-tank guns, and battledress infantry - out of entrenched mountainous terrain at a hex every turn or two in the south. Despite no Astro there, and despite me aggressively reinforcing. I've only held Burancon by filling it to the brim with defenders, but I can't put that many troops in all the mountains your mechs + infantry have been overrunning.
And you actually have things backwards about why my biker brigade is on the front lines. Me moving them there when they should be out killing wildlife and free folk was a sign of desperation when you were on the verge of splitting my northern line in half and putting half of it at risk of being cut off. Remember those wild times 4-5 turns ago? At that point, I recalled my scouts from places where they were effective, lowered their high quality standards that kept regular infantry from joining, and used now-reinforced mobile infantry as line infantry with predictable casualty numbers. I haven't replaced the bikers when they've died, and am treating that BDE as just another light infantry BDE for the foreseeable future, until and unless something changes dramatically and I can afford to move them out of the front.
I did get a nice roll with the last upgrade for medium tanks. They're frankly better than MY heavy tanks - although a big part of that was that the heavies were only 30-40% field-tested when they did the crucial jump from Mk. I to Mk. II, which is not really something you ever want unless your Mk. I is pretty exceptional. My Mk. I heavies... weren't. They were (and are) fine, but they're still in need of a lot of refinement. It's also worth noting that I put significant work into my medium tanks despite them not being particularly well-suited for any war up until the last handful of turns - I redesigned my initial awful model and thoroughly field-tested two prior versions of this one despite them being questionable for the roles I put them in. Attacking mostly infantry with a 104 or 154 soft attack score makes for slow, uninspiring field testing, but they didn't have the mobility to go where the chitinous wildlife was skulking.
Oh, and the non-front-line mechanized unit you're talking about is 100% a response to you. You've been pushing hard for the Grogs to attack my long, unexposed southern border. They've responded by building up troops on said border, which might not directly bleed me out but forces me to do what you've noted I'm doing and divert meaningful resources to border security. Having 3 BDEs that would be much better used elsewhere sit around doing PPM, SA/suicide prevention training, and basically catching up on paperwork for months on end is not something I want to do (okay, maybe the paperwork), but the alternative of dying horribly b/c I didn't is worse. They're filling the same role as all the troops you (and I) have been keeping up around the north pole. Or even more analogously, the 5 BDEs you have sitting on your western border. (Honestly, if I could effectively disengage them, my bike BDE would be a much better fit here than up north, but since they're reinforced with regular infantry they're no longer even remotely mobile.)