With the very limited hd space on those apple laptops, is there a way to use got galaxy with an external drive?
Because my current experience with using an external hd formatted as journal (or whatever the standard option may be called) just causes Galaxy to finish the download-and-installation-process with a short "installation failed" notice while the game appears in the list of installed games being unplayable.

Generally speaking I wouldn't install games to an external drive, they are read heavy, and external drives are generally slower than data speeds. What I do is ssd with os and apps, data hdd, and then backup external drives. Hdds/ssds are dirt cheap nowadays and get cheaper ea h month. Although I don't use apple, and only really use a desktop for gaming, however you should be able to either get a new drive, or just keep a minimum installed, I.e. Just.one or two games at a time.