Thank you, @v_kyr and @Old Bruce!
I created a symbolic link following the steps you have indicated. It was a bit tricky to locate the PersonaBackstore.dat file because it is so hidden, but after running a Spotlight search within "HD : private : var" I found it.
I just want to note, too, that there is a whole thread—in German, but whatever—on this same issue and the solution these lads (especially @PowerBug2.0, so thanks to you, too!) came up with was to prefer embedding files instead of linking them in the "File > Placement Policy" menu. That did increase their file size but for reasons unknown it did not exaggerate the PersonaBackstore.dat file so much.
Okay, thank you all for your help! I think this will work. If it doesn't, I will give you lads a little shout again!
DtS