Ha! The niri README has an answer for this, https://github.com/YaLTeR/niri, it is https://github.com/mogenson/PaperWM.spoon, "Tiled scrollable window manager for MacOS".
Aerospace has a similar resizing glitch as PaperWM.spoon: resizing one direction ends up looking wonky if you do it fast enough. It’s noticeable at the end of the smooth scrolling demo. That must be a macOS thing…
I may check out PaperWM.spoon at some point but realistically I’ll set up a VM and try out Niri