For me, the appeal of i3/sway's model is that by having a desktop per topic (eg, one for browser, one for code, one for slack, etc) I can instantly jump to the topic I need with a single key press. The desktops I assign never change, so it's always Super+1 for my browser and Super+4 for Slack. It's all muscle memory, and I could do it in my sleep. When I jump to that desktop, everything is open and tiled. This was a revelation to me coming from MacOS, where I was constantly hunting for windows with Cmd+Tab or squinting at thumbnails in Mission Control. So I'm surprised to hear that you prefer Niri's scroll model, which to me sounds like hunting for windows all over again.
I don't think of the things you listed as "topics". Browser, code, slack, etc.. seem more like tasks or activities to me.
I used to use i3 on a 49" super ultrawide monitor (32:9) and when I did each desktop was truly a topic, with a deep arrangement of windows on it and tabs to switch different areas over to different tasks.
My primary interest in tiling window management is being able to see all of the things that I need to see at once, at once. For me, it's all about context-- placing related windows next to one another. It seems like for others it's about being able to switch between a limited set of fullscreen windows quickly.
I like that Niri gives me a flexible way to divide my workspaces by topic without the windows stealing screen-space from each other. It doesn't feel much like hunting for windows, it feels like... a materialized view of alt+tab. Maybe I have a browser to the right of my editor and a terminal to the left. I can quickly shuffle back and forth between being able to edit while also seeing one or the other.
I even have this binding to cycle the columns to the right of the active window:
Niri has named workspaces, each of which has a scrolling model. So you can achieve something very very close to what you want.
Not OP, but I typically only have 1–2 windows per workspace. I use tabs in both browser and terminal (eg via tmux). So it seems like niri's scrolling capabilities wouldn't bring much to my use case.