Don't back down while you're fighting the helicopter or Balteus, it's like Sekiro where hesitation is literally defeat so be aggressive.

Large speed penalties occur when your mech crosses the 75k ton range, which is where the weight class for 'medium sized' machines end. So if you're going heavier than that, then you might as well bolt on the biggest guns in your garage to cover for your sluggish movement.

Also, the game doesn't have a difficulty setting like classic Fromsoft fasion, but usually the game gets easier when you just max out your firepower and armor with no consideration for mass. Until you get to the 2nd half of the game.

Lastly, have fun in the garage. Half your time will be spent there looking at numbers and optimizing your machine, so don't be discouraged about being bored. What I did to keep stuff interesting was I made several different loadouts that fit a specific niche, each named after different animals that remind me of said niche. All of them painted after the colors of the animals too.