Music Tracks folders and playlists

Before I put music into rekordbox i have folders of music on my PC and in each folder the tracks are in alphabetical order.

I would like to use one folder as an example....for now I will use this one...
It's called (OS short for Old School)
In this folder on my PC I have around 800 tracks varying from around 80-170bpm, ultimately later I will break this folder down into bpm say in ranges of 10 so 80-90bpm, 91-100bpm,101-110bpm etc etc once I have the answer to my questions as I e been trying to sort this myself for sometime now but cannot seem to get my answer, hence asking for your help.

Once I drag the folder (OS) from PC to rekordbox creating the exact folder as a playlist the same as my PC folder I then sort into bpm and then renumber so the tracks in rekordbox the tracks are now showing from first to last track in slowest to fastest bpm for that playlist.

How can I intially before rekordbox on PC know what bpm my music tracks are is as it only seems to generate the bpm when added to rekordbox, if I can sort on PC and know the bpm for each track pre rekordbox it would make things so much easier as I could then have my folders as above with breakdown in my (OS) folder on my PC having folders 80-90,91-100,101-110 etc etc and simply drag and drop the folders from PC to rekordbox this would be particular handy when I add music tracks to breakdown folders on PC and update then update playlists on PC.
As things stand on PC the music tracks show no bpm so I have no idea how to split the bpm ranges into breakdown folders, only when I drag the folder to rekordbox it will generate the bpm and I can then set accordingly slowest to fastest in order on my rekordbox playlist.
When I see these 800 tracks on rekordbox and in correct order I could (although seems very long winded) then open (OS) folder on my PC and find each song splitting them into breakdown folders as previously mentioned and as per the bpms they are now showing on rekordbox so look at rekordbox find the first track look at my (OS) folder on PC find this track by looking for it alphabetically and putting it into its breakdown folder.
Doing this initially would take forever and also when adding newer tracks I've purchased I would still not know the bpm by viewing on PC so in these case I cannot add to a direct playlist on rekordbox and would need to add to collections find the track on collections find the bpm move the track into the required breakdown bpm playlist in rekordbox find the track on PC and put into the correct bpm folder on PC which off course then means the track has moved from initial download directory to the one I want to use making it void on rekordbox where I would have to redrag new updated folder on PC to playlist on rekordbox I would have to do this for each track I add.

Please please help as this is becoming a real issue for me.
It maybe just me and I'm sincerely hoping you can help make this an e as easier process for me by advising how I should go about doing this without taking an absolute age to do.

Thank you very much.