Main-lining problem

I'm currently on my 4th grow. 2 Blueberry Yum Yum, 1 Durban Poison, 1 White Widow, and 2 Mystery seeds (both look like pure sativa)
600w MH about 14-15" from canopy
Temps are between 76F and 80F lights on
RH% ~ 60%-70%
Soil grow FFOF 3 gallon pots, no nutes yet @ 3 weeks from seed

So, here's my thing. I discovered main lining after my first grow and I've been doing it ever since. It's been an amazing way to get massive and bushy plants. There is only one thing I run into with almost every one of my plants. The nodes start alternating before I've gotten the number of mains I'm looking for!

I usually go for 8 colas total, but I find it almost impossible to get past 4. Sometimes my plants even start alternating after the first cut, leaving me with only 2 tops.. I try to top them as early as possible without stunting them, which usually ends up being around week 2 for my vigorous growers and week 3 for the stragglers..

My question to you guys is, have you ever ran into this issue? Does anyone have any suggestions as to how to get my plants to 8 tops with a proper manifold (no alternating nodes)? And lastly, what are some other good ways to keep a very flat and even canopy? I've done lots of LST but always end up with certain colas being more dominant, which makes it hard to get enough light to the rest of the canopy.

Thanks guys!

I think if you are serious about mainlining a screen can be really useful.
It is hard keeping 1 or two colas from going dominant by sticking out above the canopy without.

Eight is easy to do. wait till you have 4 or 5 nodes. Top it so you leave two sets of branches only. Then take out the 4 grow tips remaining. If you use a wide enough pot, you can stake the flowers just above the ground in a big circle all at the same height. The top set of branches are more vigorous, so they easily grow to the length to stake them with the rest. at one height.

That reminds me, I need to go FIM my sweetpeas.

I no longer main line personally, I found topping a couple times would give me a decent bush, then I'll even out and top the bush into an even canopy that usually gets an even bigger bush. I grow in troughs and beds now but when I was still in ten gallon pots i would aim for between 12-15 top colas.

Main lining is a cool concept concept but it was a little more time consuming and inconsistent for me personally.