Ok, so I have done the Import of the main Music folder.
2 things to mention:
1. I have started the import in JR and that minimized it. Result-> JRiver has closed (crashed?) after a several minutes. Every time I tried to minimize it during the import process it has closed or crashed, thus not finished the import properly. So I have to start last time without minimizing it, so my computer was occupied only by this one process and I have to wait like 16 minutes to JRiver to complete.
2. The result of the Import (just the bad side of it):
36 files skipped, because of too long filename. Ok, it's just 36 files, you would say, but as I have checked in the log there are a bunch of them that I would not want to be missing. Strange how it occurs in a software like this, extra long file name handling is generally solved under Windows systems since a very long time ago now... (Using Windows 10 Pro-> with all recent updates installed). Something that need to be fixed in JR maybe?
Regardless, thank for the tipp (tho' I'm still reluctant from using any media software's library building options), but I will see how it works for my expectations in practice later...