The grind isn't about training hard enough, it's about consistently training. Doesn't matter which mix of exercises someone decides to do (it does, but assuming the opposite is zero), consistency will eventually become a grind.

You're right the main enemy is quitting. When I tell people I workout nearly every day, people have this illusion that it's easy. It's not. I make time for it in my day, I make it a priority. That's what I mean by the grind. Everyone is different, but I find embracing the uncomfortableness easier than looking for ways around it.