I lift 5 days a week and do cardio 6 days a week. I'm 48 years old and today I did a half hour cardio in the morning and pulled a 455 lb deadlift at lunchtime. You're not going to hamper your gains by doing cardio after leg day, you're just going to get fitter and stronger, but you really have to lean into it and trust the process. Doing it halfway is just going to leave you tired and sore and with nothing to show for it. Give it a big "ALLAH ALLAH ALLAH", bust your ass, then get a good night's sleep. Rinse and repeat.