I was using Karabiner a lot at the time. I hadn't heard of Rectangle nor Hammerspoon, but I tried Amethyst (or something like that) which would provide a tiling experience, but I hated it. It was laggy, and I couldn't get used to the windows jumping around. Maybe it was my oldish MacBook (late 2013 15" MBP), although I still find that computer plenty fast for my needs.
I was more or less in the same boat app-wise, as for work I use Office 365 and at the time the web apps were barely usable. I also loved Apple Mail, and to me, it still is the best email client I've ever used (disclaimer: I don't tend to do anything "fancy" with my email). Nowadays, Outlook online works well enough for my needs (I'd say it's actually better than when I was using the 'heavy client' on the Windows machine I was given). Teams works well (as in no worse than elsewhere) on Linux, too, so all my work-related needs are met.
Rectangle is just an app that helps to move windows around, like on Windows: Win-RightArrow to move to right half of screen, some other combination to maximize etc. I found that recreating i3 experience as-is on a Mac is counterproductive, especially when it's on a laptop-sized screen. I rarely want to split the screen, and when I do, it's mostly left-right halves. The only exception is terminal, and I use tmux for that.
Hammerspoon is just an automation tool for Mac, you can do a lot of things with it, including moving windows around and a lot of other stuff (just look at the docs to get a feel). I use it as an app switcher: I found that it's more efficient to do "show me Slack" or "show me VSCode" than "move to workspace 3 where VSCode should be". I don't use workspaces at all nowadays. So, with the help of Karabiner, I mapped my most-used apps to combinations like "caps+g" for Google Chrome, "caps+s" for Slack, "caps+c" for VSCode etc. And for cycling through windows of the same app, there's a built-in cmd-` on MacOS.
I was more or less in the same boat app-wise, as for work I use Office 365 and at the time the web apps were barely usable. I also loved Apple Mail, and to me, it still is the best email client I've ever used (disclaimer: I don't tend to do anything "fancy" with my email). Nowadays, Outlook online works well enough for my needs (I'd say it's actually better than when I was using the 'heavy client' on the Windows machine I was given). Teams works well (as in no worse than elsewhere) on Linux, too, so all my work-related needs are met.