I can heartily recommend BattleTech Advanced if you don't want to go as far as Roguetech. It's definitely tougher, but not as impenetrable as Roguetech is. (Plus there's lots of difficulty sliders so you can probably tweak it to be about as forgiving as vanilla).

I really enjoy the MWO-style mech customization that both of these add (the https://github.com/BattletechModders/MechEngineer mod); one critique I have of vanilla Battletech is that they simplified the mech construction mechanics a lot, and stripped out some of the tradeoffs that make mech design interesting in the canon. In vanilla, you just optimize for free tonnage, and can put jump jets on everything, so there are just a few chassis in each weight class that are strictly optimal for most roles; this makes it quite boring as there's little reason to pick other chassis. In BTA/Roguetech you can take an XL engine to shave off a lot of weight, but that means your mech will die if you lose your side torso. Lots of meaningful trade-offs, with no "best solution".

I will admit that my eyes glossed over when I first started the game and looked at all the new part types I had to learn though...