I can tell you that I use my iPP 11 for probably 90% of my photo and video editing these days. I can’t say that it’s my ONLY device, since my PC is still required for some heavier work, however the iPP has replaced most of the work I used to do on the PC. I shoot a lot of photos and videos for my work and the iPP has saved me a ton of time in getting content finished.
Now if Apple would unleash the iPad to its true potential by giving it a real file system and real multitasking, then it could very well replace my PC for those tasks. I think it’s a tragedy that the M1 is saddled with those ridiculous limitations on iOS/iPadOS when it can very clearly handle high-end desktop tasks easily.
The only factor for the 12.9 vs the 11 for me would be whether I will be using the iPad mostly at home (or similar environment) or a device I will be carrying around very often. I considered the 12.9 but it’s just too large to carry around regularly and easily pull out and use, but I will confidently say that I have never felt that I needed a larger screen when using my 11.