Addition: after several contacts with support, it was confirmed to me that it is actually the case that the benefit package always comes into play when it is renewed, even if a purchased package is still running! Although I am very satisfied with Swisscom’s services, I suggested that I “take a little grain of salt” on this point, because that doesn’t correspond to my logic.
As already mentioned, I can live with that because 500 MB/30 days is usually enough for me. If necessary, towards the end of the period, e.g. when there are only three days left until the renewal, I can deactivate the mobile data traffic and switch to WLAN, if necessary also via hotspot on my wife’s cell phone 😂, or at the “snail’s pace” 128 kB so far possible.