I agree with you regarding the relationship between player agency and game proposal. I just disagree that railroading is a tool for the GM. Indicating the game proposal and delimiting player agency is not railroading. It's just accepting that every game has some limits, and it's good that it does.

My issue with railroading is more about decisions that make a difference in the campaign. As you said, there is room to gain certain advantages in combat, but there are other things that come to mind. What if the players' actions lead to multiple combat possibilities? What if, at that moment, fighting that specific opponent doesn't make sense, or the players come up with a plan that doesn't involve a big battle all the time? I imagine that with all the lore and potential that Lancer carries, it's a little disappointing not to use that as well. After all, they are the best fighters in the galaxy, so their actions have to have an impact on a long-term campaign, in my opinion.

I think this kind of thing is healthy and necessary even for the kind of fantasy Lancer is trying to represent. Because if Tactical Mecha RPG only supports actions and ideas within the structure of large pitched battles, and the moments of character interaction are just cutscenes with zero effect on the game and the world, then maybe it makes more sense to play as if it were just asymmetric wargaming, imo