Hello, Whitney, and welcome back to the forum 🙂
I have advanced tremors and know the struggles all too well. My wife and I have had to develop unorthodox techniques to overcome our handicaps. We shoot birds and macro hand held because we find monopods to be to restrictive when tracking birds as they flit around in the canopy and inconvenient to pack around when on macro outings, but still manage a large number of keepers. This is largely due to the newer R series cameras IBIS, RF glass with IS, the cameras Digic X processors which allow us to use higher ISO and therefor higher shutter speeds in the lower light conditions in which our subjects are usually found. We have been shooting since the '80's (35mm film) and old habits were hard to break (low ISO low ISO low ISO!), but these newer cameras handle high ISO (between 800 and 4000) so well it's given us an advantage over our conditions by allowing high shutter speeds to help counteract the tremors.
I also like the idea of the EOS R7, if you want to stick with a crop sensor and if you have budget restrictions. I like full frame, so I chose the EOS R5 and my wife chose the R6 and has upgraded to the R6 mark II. I use all RF L glass (5) and my wife's primary lenses are the RF 100-400mm and the RF 100mm L macro. Her rig is so light it's hard to believe, but my R5 and usually the RF 100-500mm L is a bit heavier but still quite a bit lighter than my EOS 5D mark IV and EF 100-400mm L that I upgraded from. For some reason, the lighter rigs ease my tremors, I guess because I'm not having to grip so tightly. At any rate, all of the above reasons make it possible for me to get a lot of keepers (lighter rig, IBIS, ILS, higher ISO, fast shutter speeds), it just works for me, and trust me, I shake really bad and sometimes am amazed that I got a sharp shot at all, let alone 8 out of 10.
Now, take into consideration that if you intend to keep your Sigma glass, you will need an adapter and there is a possibility they (Sigma) will not work as expected with which ever R camera you purchase. Some have reported problems and some seem happy. But I can tell you from experience that RF glass is where it's at and will give you the best IQ that the camera is capable of. I've used many adapted EF lenses, including third party, and it didn't take long for me to switch them all to RF, but I am quite demanding. Hopefully, your 150-600 will work well, but I would definitely replace the 105mm with the RF 100mm f/2.8L macro as soon as you gather the funds.
I hope this helps, and feel free to PM us if you have any questions about how we shoot to overcome tremors.
Newton
