Bulgarian split squats! You're going to have to lower the weight and regain that mind-muscle connection as well. It may feel regressive like you want to hit those heavy loads again but you have to play the long game. You'll be rewarded for it.
Especially since you have lost some stabilizers that could dramatically affect your biomechanics, so use light weight (probably start with body weight) and just get the feel for how the leg mechanics work now. Pause reps are really good for that, like pausing for 5 seconds at the bottom, halfway up, etc.
Diets are just a restriction of a certain food group or a reduction from your normal calorie intake. Their purpose is to lose weight and is short term.
If you're thin as you say, then just go to crossfit for a couple months and you'll gain a bunch of muscle and probably be shredded. No need to diet at this point, in fact you might have to eat more to sustain the muscle growth.
Like I'm not stick thin, but I'm not as defined as I'd like to be, basically.