I just use Window Maker. It got an update recently. Notifications work out of the box, Firefox and Chrome have never created multiple icons, not seen that.
It is not a Wayland compositor which is fine as I only use X11 and probably won't use Wayland for many more years till it's mature enough. I went back to Window Maker several years ago and it's working just fine. With wmsystemtray
I have a system tray so things like NetworkMakager and hplip and blue-z all can latch on and display their icons, I don't need a desktop environment now!
YMMV regarding the HIDPI thing, I have never had a monitor with such a narrow pixel pitch to need anything like that.
GNU Parallel can also run jobs across the network on other machines as well as the local CPUs