Final progress note: Just did a zypper dup (-> new kernel) with the selfcompiled version installed. Builds and installs the module for the new kernel correctly, and this time did not touch the previously compiled ones of the older kernel. (and I just realize that the reported removal of the 4.15.2 modules was of course because they were part of nvidia-gfxG04-kmp-default-390.25_k4.15.2_1-10.1, which gets removed by an update....)