I can give you my experience. I used Rebelle 5 and 6 Pro versions with a Mac Mini M1, 16GB of Ram. I did dozens of artworks in both versions since then.
Both versions run with Rosetta (the software layer which allows to run Intel (Mac) software. They always worked pretty well and fast enough.
I can assume Rebelle 7 will run even more faster. So my 2 cents is that M1 is enough. Note that i also run other software both Apple Silicon and Mac, this computer it's still pretty fast on averything. Currently the software i run, almost everything is already Apple SIlicon; the software that is still Intel is older software. So i guess it's a good time to change to M series.
My advice is that M1 is enough. Just make sure to upgrade to at least 16GB of RAM. Make sure to buy a Mini or MacbookPro M1, the Airbook don't have fans, the performance may be lowered with intense use.
For CPU you will not have any problem. For GPU, it depends if you wish to run 3D software. In that case, better Pro or Max... but i guess it will be a good idea to stay with a base M... and use the money you don't used pay for Pro/Max to purchase a PC with a dedicated GPU. About Rebelle, you will not have any problem with the M1 GPU on any of the Rebelle features.
An important note. Currently, all native Apple SIlicon software is Universal. This means that Rebelle 7 will be Universal too, and it will have both versions, Intel and Apple SIlicon. I recommend to Pre Order Rebelle 7 before november 30. You will be able tu run Rebelle in your current Mca, and also benefit when you upgrade to Mac M.
For A4 300 dpi, you will have no problem at all with NanoPixel on any modern mid range computer, it's a good size. However, i don't recommend to use NanoPixel when you paint. I didn't notized any slowdown with my Mini with NanoPixel ON/OFF (you will not even hear the fans on M1 lol). But it's a good practice to not use that feature you will have to export to NanoPixel.
Lastly, i agree with you. This version looks very promising. Escape Motions team added a lot of things the past years, and 7 seems to have even more. ALso, very nice pre-order promotion. It's a "go to Pro version NOW".