Right now it seems the user is forced to use the key combination (default Meta+G) to activate the desktop grid effect. This key also works to transition from the overview to the desktop grid. An equivalent trigger should exist for the mouse only. And indeed, you can transition from the grid mode to the overview by using the hot corner feature, but the opposite isn't true.

I'd suggest to use the scroll wheel while in overview/desktop grid for transitioning between one mode and the other, as the scroll wheel seems currently to do nothing in this effect. An alternative would be to let the user to configure an hot corner shortcut to directly access the desktop grid effect, since this is not possible nowadays.

Using the scroll wheel for transitioning between the desktop grid and the overview may also be used to switch from the desktop grid to the overview of the desktop the mouse is hovering over. This way the user can switch using the mouse from different overviews to organize their windows, without being forced to exiting and activating the effect over and over. Right now the only way to switch from different overviews seems to be using the arrow keys, which seems to be pretty inconvenient.

Adding this feature would also contribute to address from a functional perspective this bug, https://bugs.kde.org/show_bug.cgi?id=475077 giving an alternative to users used to the linearization of the workspaces that we had in the previous Plasma version.