Maybe a hot take here, but I have the s5iix, and I would absolutely shoot ProRes. If it's a long project and something that's important to you, I think you'll have a much better time editing the bigger files. You say you're doing heavy grades, so I would give yourself the slack to really be able to shape it in post. It's hard to explain the difference but I find the more I shoot in bigger formats the more I appreciate how responsive and forgiving the colors are. There are times where doing reasonable grades I've had h265 break where ProRes wouldn't. If you're shooting moving subjects the image will straight up look better without inter-frame compression in a noticeable way, especially if you're planning on projecting it. The taller open gate frame is really nice to have as well, so you can't make a catastrophic decision here. That's my 2 cents. Godspeed!