Hi, I did want to add a little to this conversation based on my recent experiences with the Sigma 60-600 & the Lumix 100-500. I am not a Youtube blogger or "lens influencer" so this is just my personal experience based on personally purchasing both of these lenses and making use of them. I have an S1R2 and the S Pro 24-70 (Japanese version) & 70-200. I am a big fan of both Sigma & the S Pro lenses that I have.
My primary use for the longer primary telephoto that I wanted to add was to photograph the barred owls that I share a yard with. We have a nesting box in our yard and it's a yearly routine for them to have babies, etc. I typically shoot them with a GH5M2 with a metabones converter with a Canon 70-200 which essentially gives me 400mm/2.8 which is hard to beat. The images in low light are quite useful. But I also wanted to make use of the abilities of the S1R2 as well so began exploring lens options, aware of the slowness of them.
To me the Sigma 60-600 seemed best on paper and I initially purchased this lens. I thought I could handle the weight and had hoped to do handheld work with it. I did a lot of test shots but was just stifled by the zoom ring being at the end of the lens and a few other features. But, it did take some great images with a gimbal & tripod.
I then started reading & watching all of the glowing talk about the Lumix 100-500 so I returned the 60-600 and purchased the Lumix. I was pretty impressed with it at first. Much lighter, easy to handle. And maybe I just had a bad copy, but i was just not happy with the results form 350-500 and I eventually figured out that it took much sharper photos if the subject matter was within 20' or less. And then I did the same shooting tests with it that i did with the 60-600 and really not nearly as sharp at 500 in comparison. I actually started realizing my 70-200 photos cropped were more preferable to me than the 100-500 images as well.
Unfortunately my experience w/ the 100-500 was the same in regards to birds. I also returned mine. I ended up getting a Canon as a second camera for wildlife but now realizing I don't use it quite enough to justify a second. Funny, I *also* tried out the 60-600 (before getting the 100-500) and it was super sharp and maybe one of the fastest focusing lens I tried IIRC. But the weight imbalance at full extension is brutal. Perhaps I need to revisit it as well though. A longer foot is a great idea. The stock foot on the Sigma is *way* too short. I was hoping Sigma would refresh the 150-600, which I find slow on Lumix cameras, but no news so far.