Would a manual reclaim be possible like this -> Player clicks first stone/mass and holds the mouse button, then moves the mousecursor to next stones/mass, these gets marked without an andditional click.
Manual Reclaim with just one click
The game is done deliberatly in a way, that there is no other way as effective (and easier) as manual reclaim, clicking on all rocks. https://forum.faforever.com/topic/3846/reclaim-brush
There was discussion recently regarding a similar approach, and the common consensus was that it was either unnecessary or deviated from the gameplay mechanic a little too much.
Also the recent random UI improvements mod adds a new reclaim mechanic that may work well for you.
@zlo I'm using it and haven't experienced any bugs.
It's just weird, that the reclaim command is still active if you let go of shift. So you have to click some empty spot or something or you will reclaim whatever you want to select next.