It sounds like you have your heart set on a specific sensor size more than any specific capabilities.
What camera are you using right now? What do you like about it and what do you dislike? What do you find limiting with your current gear that you are hoping to improve upon? Are there any types of photography that are a priority (wildlife, portraits, sports, landscapes, etc)?
My best camera right now is the Canon G3X and Panasonic FZ2500 (both are 1 inch sensors) - I guess the G3X makes me lean towards the Canon M3 because I'm quite happy with the G3X.
Those are both reasonably competent cameras, so that helps frame the discussion
The next logical step would then be larger sensor, something like a M43 or APSC, but really as I mentioned above, I am more of a hobbyist (I just love to try things out) - but at the same time, I don't really want to spend too much.
I want to know what am I getting from an APSC that I'm missing on the 1-inch, how much better is low light or night shooting compared to a 1-inch, is the image quality noticably better (stuff like that).
I'm into superzooms mainly (wildlife) and I have a few converters (I hope they can work with the m3).
So roughly if it works out, I'd have an APSC camera with equivalent focal length of 320mm + a 1.7x TC would give me 544mm equivalent. (I don't mind spending $400 to test this, anything more I'd think is too expensive)
G3X is 24-600mm f/2.8-5.6
FZ2500 is 24-480mm f/2.8-4.5
Proposed M3 kit is 29-320mm f/3.5-6.3
Adding the 1.7X teleconverter takes you to 544mm f/11
While the sensor is the M3 is bigger, the lenses are slower (darker). The sensor in the M3 was also a bit behind the times compared to the Sony sensors in your current cameras. Best case, you will see a one stop improvement in image quality over your current cameras. In other words, ISO 3200 will now look like ISO 1600. Worst case, there will be no perceptible improvement. The autofocus in the M3 also won't be a noticeable improvement over your current cameras.
If you were talking about the M6 II and an adapted EF 100-400mm f/4.5-5.6 IS L II, that would be a massive improvement over your current gear in all regards. But that kit is eight times the cost of the M3 kit. If you want a significant improvement in low light capabilities, you would need to add a lens like the EF-M 22mm f/2.0 or 32mm f/1.4.
The $400 cost of the M3 kit is relatively inexpensive if you just want to play around with some different gear. Just don't expect any massive performance improvements. As a low cost alternative, I would maybe suggest finding an EOS M100 and EF-M 22mm f/2.0 lens. This would give you much better AF and low light capabilities compared to your current gear. It would complement your current cameras instead of roughly duplicating your current cameras.
