The 600mm is over 13 pounds.
It is nearly DOUBLE the weight of the latest 500mm f/4L IS MkII !!!
Just for reflection, i will tell you I went through a similar step by step progress for my bird photography. At one point I went from a SIGMA 50-500mm "Bigma" up to a 500mm prime with no IS. There is no question that a big fast prime will help you make images that stand out from the slower zooms!
The differences here though are note worthy.
- That old 50-500mm though a good way to get your feet wet, was the original Non OS with middling sharpness and image quality as compared to other lenses available. Your SIGMA 150-600mm is a VERY different beast, with IQ and sharpness that would have been called "Prime like" back in the early to mid 2000's. You really aren't missing a lot with that lens. (exceptions, light gathering and subject isolation)
- The 500 Prime I went to was the SIGMA 500mm f/4.5 EX, which among fast long telephotos, is one of the lighter weight options. It was a great lens, and I used it for some time and truly loved the image quality and look iof a fast telephoto prime. But what it really taught me was that I wanted longer, and I wanted IS. The SIGMA was pants with a 1.4x Teleconverter. A Canon f/4 with IS would solve that. And it did. that was my next step, and I stuck with that lens for well over a decade.
IMHO, having shot with long lenses with and without IS/OS, both slow zooms, fast zooms and fast primes, my opinion is that 500mm is at the far reaches of what I'd consider to shoot without IS.
My feeling then, based on my own experiences is that one of the Mark 1 IS models would be a better option for many reasons. And given the weight of the 600's, I personally have stayed at 500mm. An f/4 will still offer a superb 700mm f/5.6 with a 1.4x t-con. And 700mm with IS is sweet!
I'd strongly suggest skipping the non is step, and use what you have and go straight to an IS model when you can afford that.
Now of course there are some that do not use IS, turn it off in fact. Maybe you'll end up one of them?
The mention of tripods is important. 16 pounds of gear needs a beefy tripod to shoot (for most of us anyway) Your looking at an additional $1K for a good set up for a 600mm.