That does make sense tbh, I was thinking that 1200 lines won’t go into 2160. I feel like 720p should be good for NTSC (240p x3 as you said) and then for PAL, the settings I shared should be good, as it’s reducing 288 to 270, then 270 x4 = 1080, 1080 x2 = 2160p. It shouldn’t lose any detail as a lot of PAL games have black borders to some extent.
EDIT: 5x does definitely look sharper though to me, though that’s using 320×240 mode without checking out the exact settings. It’s just slightly softer in 720p is all, though it should be nice and uniform. The main difference is scanline spacing.