Weight lifting question

I was wondering, I know variance is good to keep your muscles stimulated, and a routine isn’t something you want to settle into.
So, would it be beneficial for me to do 1 day of say, heavy weights, so I can only do 3,4,5 reps, and then the next workout do weights I could do reps like 15-20 on or so?

It is a good idea, but you dont need to vary it so much. You could do one week of 5 sets, then 10, then 15. You want to keep a fairly static number of total reps, so adjust the sets accordingly (5x6 sets, 10x3, etc.). You also should vary your rest period between sets. You can go from 30 seconds rest to 3 minutes. Personally I like short rest on light weight sets and long rest with heavy weight. Try different stuff and see what you like best.