So, you're kind of doing what Martin Berkhan refers to as fuckarounditis. If you're lifting heavy things, why are you running and kicking things? Intervals have no place in lifting heavy things. They're fine for HIIT, but not for LHT.
LHT should be focused on big compound movements and only those movements. It's not a conditioning session. I'm an iron hound, I love squatting and deadlifting, but you can easily do a great LHT workout using only bodyweight. But focus. Stop messing around.
LHT should be focused on three things (maybe four): a push, a pull, and a squat. Hinges (deadlifts) are also good. Your squats and deadlifts will firm up the butt.
TRX is fine, if you're using them for pushes (dips, ring type pushups), pulls (body rows/aussie pullups/actual pullups). Do some squats or modificiations thereof (tuck jumps, box jumps, one legged squats) and you're 90% there.