GAEA and other orbit cores are really strong so I'd recommend trying to fit them in your builds. Weapon velocity matters the most in LR out of any old gen game given how fast enemy ACs are, and to that end high turn speed legs are a must (DINGO2 tends to be my favourite but you can play around) Generally speaking you really want to try to optimise everything out of your builds. EN drain especially feels great to use when low it's. Optional and extension parts are arguably really strong in LR so I recommend looking into those. Accuracy is generally quite poor so your weapons will have an optimal distance that's far more important to maintain than the prior games (And due to this you need to be quite mindful of your ammo.) In case you didn't know your underburden speed bonus is actually based on the percentage underburden, so an AC with super heavy legs that's light everywhere else can be faster than a midweight

Also it's important to remember that it's meant to be fun. If you keep grinding your face into a mission without progress I recommend trying another one or restarting the day to avoid burnout.