I get pumps with low reps (3-5 reps) and heavy weight (85-95% of a 1RM). I do a lot of volume with rest periods just long enough to where I feel like I can lift the weight explosively. I don't time myself but it is anywhere from 1 to 3 minutes I would imagine. As far as volume, I may do 20+ sets on a good day, but nothing less than 10 sets unless I feel bad in the gym that day which is very rare. I personally don't train for size (even though I built some muscle in the low rep range) and I certainly don't TRY to achieve a pump but I still get them using low reps and fairly high volume. My goal in this low rep range is purely strength and work capacity. If size is your goal then I certainly don't think you need to LIVE in the 6-8 rep range. If I could make a suggestion, maybe do your main compounds in the 6-8 rep range (personally I would prefer 3-6 but that's just me) and just focus on getting stronger at those lifts. Then after that main lift add in some structural work in a higher rep range with different exercises with a focus on getting a good pump with those lifts.

pump means very little, just blood rushing to your muscles. Periodised training is what you should be concerned with. In other words, trying to add more and more load to your training/ weight you can lift. In your first year of lifting this is relatively easy, first few months it is possible to add weight or reps to every work out. After that you may switch to just adding load weekly until even then you cannot add load, then the periods get longer. Planned Periodised overtraining. This is how all natty training works as long as you have a spot on diet. Not sure how training on cycle works, I assume it works the same except you can recover more and put more load on + add more muscle mass, in a much quicker time.

I love low rep-heavy weight training, but I rarely feel the pump too. For me, it's about the exhilaration of lifting more and more weight. I like training more like a powerlifter so pump and muscle volume isn't my main concern, but if you're doing bodybuilding, then it's pretty necessary. In which case, you should indeed do both low and high rep stuff. On days where I do try to get a pump out, I like to warm up, GO HEAVY, then drop it down and work in some volume on the reps.

You need to do both high reps and low reps for maximum growth

Just my 2 cents but I think you should try to vary rep ranges to get the most benefit. Try doing heavy one week 3 - 6 reps and lighter the following week 8 - 12 reps or even 12 - 15, in fact you could break it down into 3 weeks, moderate, heavy and light. Working this way will ensure over the whole 3 weeks you will hit all muscle fibres, your strength will increase with the low reps and thus you can use a heavier weight for your pumps on the moderate rep range causing most muscle hypertrophy.