European Goldfinch on a tree next to the parking lot of my office (North-Western Germany, July 2023)
I have the same lens. It's very versatile. I'm hoping Canon let 3rd parties use their RF mount. Sigma have a 150–600mm lens that would be very tempting if I could get it with the RF mount.
I am not sure if you should bother with the Sigma. I had a lot of trouble with my Tamron 150-600 mm lens and I've heard of people who've had similar problems with Sigma. Issues with focussing and sharpness especially. And it's also really heavy. I've been able to get better pictures with the 100-400 than I was with the 150-600 mm Tamron which I've ultimately sold again.
I would recommend you try it out before you buy it. Maybe mine was just a bad model. But as I said, others have reported the same thing with Tamron and Sigma on Canon R cameras.
Might give that a miss for now then. The person I saw raving about it had an old Canon DSLR with an EF mount. I've got the 800mm prime Canon RF lens, but it's the 'affordable' one (as opposed to the one where you have to choose between a lens or a downpayment on a house). The lens is great, but the high aperture limits its usability to bright days.