A 27" 4K at HiDPI 2560x1440 looks perfect. No reason not to use this resolution. Yes, it's a "scaled" resolution, but the M1 slices through it without a single stutter.
I'm using a 32" 4K at 3008x1692 and it also looks amazingly sharp and the M1 never drops a single frame.
Ditto 4K scaled to look like 1440p. Looks great. New GPUs should be fine with it.
Another +1 for using a 27" 4K and running at scaled 2560x1440. I've been running this setup off my 2016 13" MBP for almost 5 years now, and it looks great. As other have said, macOS still pixel-doubles this effectively to 5120 x 2880, and then downscales it to fit the 3840 x 2160 4K screen. Honestly, I know a true pixel-doubled 27" 5K for 2560x1440 would be better, but given the lack of 27" 5K screens (and even when you can find the LG Ultrafine), at a fraction of the cost, I think it's a great bang-for-buck value.
As others have mentioned, the other consideration of doing this is that it's more taxing on the GPU for non-integer scaling. But has been fine on my 13" MBP with pretty old Intel Skylake integrated graphics (though admittedly I don't do much graphics-intensive work). If you're going to pair it with a newer M1 MBP (or an M1X 14"/16" MBP), I think shouldn't make a difference.
