Apple Music Era (fiasco in your words) was a bit of a joke, that would be from Feb 2020 - May 2020 for you, the time when you had the Apple Music account active.
The music origin was just me trying to find a way to recover the music through other methods. Based on math using your numbers above, you should have 3800+ tracks that are yours. It is not feasible to try and get the 750 again, you would be almost 86 when that was done. The CDs could be done, they would just take some time, and that is the bulk of the music.
I am surprised to see that you have 3 copies of iTunes running and 3 iPods. Most people can barely handle 1 copy of iTunes and try not to use it. I would guess you provide technical support to your neighbors and friends too. Good to be one of the knowledgeable ones.
Song file location is in D:
This would be the cause of the exclamation points, as you are probably testing after removing the thumb drive, which is the right way to test.
You are not impacting the other computers at all.
Let's check your settings on Import of new music.
In iTunes, click on Edit->Preferences, then click on the Advanced tab.
In the top half there is a big box for your iTunes Media folder location, under that are 3 check boxes. You need to make sure the first 2 are checked.
Keep iTunes Media folder organized will move media files into <Artist>/<Album>/[<Disc number>\]<track number> <song name>.<extension> in the media folder.
Copy files to iTunes Media folder when adding to library will make sure when you drag and drop files from the thumb drive, they get copied to the computer hard drive so you don't get the exclamation points after you remove the thumb drive. This I think is not checked for you, causing the thumb drive copies to not work properly.
Click OK to save the setting changes.
Question about your other iTunes installations:
Do these other copies of iTunes play your personal songs and have your playlists?
If the answer is YES
Put the thumb drive into one of the other computers (not main laptop)
Using File Explorer, navigate to C:\Users\<login>\Music
Right click the iTunes folder and chose Copy
Using File Explorer, navigate to the thumb drive (make sure you see the songs in File Explorer)
Click on the Paste button (Windows 10) or Click on Edit->Paste (Windows 7) or hold the CTRL key, type V, release the CTRL key to copy the folder to the thumb drive. Wait for the copy to complete.
Go to your main laptop
Close iTunes
Using File Explorer, navigate to C:\Users\<login>\Music
Right click the iTunes folder and chose Delete to send it to the Recycle Bin.
Using File Explorer, navigate to the thumb drive
Right click the iTunes folder and chose Copy
Using File Explorer, navigate to C:\Users\<login>\Music
Click the Paste button, wait for it to finish.
Open iTunes
Playlists and Music should be there, everything should play.
If the answer is NO:
The songs can be recovered but playlists will have to be created by hand.
Clean up previous imports:
Let's create 2 playlists in iTunes:
Create a
normal playlist, click on
File->New->Playlist, name it GOOD
Select all the Music in your Music section (Click on any track then type CTRL-A), drag the selection and drop on the GOOD Playlist.
Create a smart playlist, click on
File->New->Smart Playlist. Set the criteria to Playlist is NOT GOOD. And click OK. Name this playlist BAD.
BAD will be the list of all songs that iTunes can't find.
Click on the BAD playlist, click on the first song, hold the CTRL key, type A, release the CTRL key to select all the songs in the BAD playlist. Right click one of the songs and choose
Delete From Library to remove all the songs that were not copied correctly.
Add music from thumb drive:
Open
File Explorer and navigate to the thumb drive. Move this window to the left side of the screen, and resize so it only fills 1/2 of the screen.
Open
iTunes, move this window to the right side of the screen, resize so it only fills 1/2 of the screen.
In
File Explorer, click on the first song, then hold down the CTRL key, type A, release the CTRL key. This will select all the songs,
Drag the songs from
File Explorer and drop onto the
Library section of
iTunes in the top left corner of the left column.
Wait a minute or five for the copy of the files to move from the thumb drive to iTunes, you can watch the progress in the top middle of iTunes.
Now the songs should be in iTunes and play.