Honestly pretty much all rotary tools kind of suck these days. I have destroyed two m12 tools (both times metal dust shavings made their way inside from cutting steel) and I now have a dremel because I couldn't bring myself to buy another m12. I haven't had it long enough to know how it will last. I have borrowed an m12 die grinder and it was pathetic. You barely lean into it and the thermal overload kicks in and shuts it down. Over and over again during the whole 3 minutes worth of battery life with a 6ah. For occasional use I would just buy whatever amazon is selling at this point because there doesn't seem to be any (brand name or not) that are actually built to high quality standards.
If you want the "best" rotary tool check out Foredom:
https://www.foredom.net/product/k-txh440-industrial-kit-with-square-drive-shafting/
What tool were running in that die grinder? If you say a cut off disc larger than 1.5", then that would be silly. It takes some serious horsepower to run a 4" cut off disc with any efficiency whatsoever. I have a right angle die grinder that does it. The average person won't spend what it costs nor acquire the large air CFM required in a compressor to run it. Mine is about 14 cfm at 120 psi and barely, and I mean barely keeps up.