I never had an issue lifting 3 days a week and doing cardio 3 days a week. I’m just changing my cardio to cycling. Been doing that for 2 years. You get to know your body and how it recovers. So my workouts are geared towards time and intensity in line with what will be sustainable.
I agree the importance of not overtraining, resting, eating (my favorite ; ) and if your feeling like you need a break, taking rest days when needed. 6 days a week of excercise is a very realistic goal. Even 7 on certain weeks. I’m in it for the long haul, if I miss a day...always go get em tomorrow ; )