yes after a reboot stuff gets reopened in the same place it was (sometimes some windows do not, is not 100% stable). but only windows that support saving themselves into the session get reopened, stuff like vscode or Spotify do not. Firefox and kde stuff yes.
Firefox is very stable in this regard, it always reopens all the windows with all the tabs in the right desktops.
the activity part is definitely a bug, its a kde + X bug and it is in wontfix state. i hope they will rework this feature on plasma 6 with Wayland
the mouse part is the part i like most. usually people use this kind of workflow to not use the mouse… i did the opposite. in kde with meta+left click you can move windows between monitors or to reorder them in the tiling. and with meta+right click you can resize them. this means that opening 2 windows will open them with half screen for each (because of the tiling) but with meta + mouse i can reduce one window (and thus enlarging the other), it’s very fast and very convenient.
the 20 desktops are a lot but i don’t use them all, i generally organize my work in rows. but sometimes i use the desktops differently and i like to have that kind of flexibility.
i actually used 2 activities to separate work and personal, with 2 separated Firefox profiles. so i had 40 desktops for personal and 40 for work… :)
but activities have their own set of problems, like, there is no shortcut to send a window to another activity, you have to do it from the menu in the panel. and after a reboot, sometimes windows get thrown into the wrong activity, and that’s very annoying.
to add a bit of context, i’m a software engineer, and the combination i use most is vscode on one monitor and Firefox in the other. with maybe dolphin on a neighbor desktop. this repeated 4/5 times depending on how many projects i’m working on.
i have 2 screens, i use a grid of 20 desktops for each screen. the grids are synced between the screens, if i go left on one screen, it goes left on the other
i have tiling; i use bismuth to add the tiling to kwin. i have set up shortcuts like meta+f makes a window float etc
i have an mx master mouse with the thumb button and other configurable buttons. i have logiops to remap those. clicking the thumb button will bring up the desktop grid. thumb+up goes a desktop up, etc… this is extremely comfortable to use
i have configured the task manager in the panel to only show apps opened in the current virtual desktop. this way i can have a Firefox for each row for example.