This is what us proles on third-party monitors have to do to make text look halfway decent. My LG DualUps (~140ppi if I recall) run at 2x of a scaled resolution to arrive at roughly what would be pixel-doubled 109ppi, which is the only pixel density the UI looks halfway decent at. It renders an 18:16 2304 x something at 2x, scaled down by 2.

It's also why when you put your Mac into "More Space" resolution on the built-in or first-party displays, it tells you this could hurt performance because thats exactly what the OS is going to do to give you more space without making text unreadable aliased fuzz, it renders the "apparent" resolution pixel doubled, and scales it down which provides a modicum of sub-pixel anti-aliasing's effect. Apple removed subpixel antialiasing a while back and this is the norm now.

I have a 4K portable display (stupid high density but still not quite "retina" 218) on a monitor arm I run at, as you suggest, 1080p at 2x. Looks ok but everything is still a bit small. If you have a 4K display and want to use all 4K, you have the crappy choice between making everything look terrible, or wasting GPU cycles and memory on rendering an 8K framebuffer and scaling it down to 4K.

I'm actually dealing with this right now on my TV (1080p which is where I'm writing this comment from). My normal Linux/Windows gaming PC that I have hooked up in my living room is DRAM-free pending an RMA, so I'm on a Mac Mini that won't let me independently scale text size and everything else like Windows and KDE let me do. I have to run it at 1600x900 and even then I have to scale every website I go to to make it readable. Text scaling is frankly fucked on macOS unless you are using the Mac as Tim Cook intended: using the built-in display or one of Apple's overpriced externals, sitting with the display at a "retina appropriate" distance for 218ppi to work.

Pedantry: 18:16 is the same as 9:8 since it's a ratio.