the simplest most glarring solution is....
6 hours of music if compressed on average ought to end up being less than 100 songs. at roughly 5MB per, that's only half a gig of music more or less.
I had a 16GB phone and know the issue very well. you should be able to carry 2GB or maybe more on your phones, since they are not cluttered up with games or other non essentials.
that comes to several hundred files.
you said you bought them all from Amazon.
meaning you have an account. meaning you also have a history with Amazon. even if you don't have the compiled CDs Amazon ought to be able to allow you to redownload them.
if so, just load them onto your phones instead of the laptop.
alternatively....
I would find out what my car can if at all, do with that USB jack, or some other hidden jack there may well be in there somewhere.
does the car have a radio in it at all? there might be a way to tap into that via RCA/Headphone jack, or simply via BT from a small portable media player.
a friend's new Chevy came with Cirrus or XM radio.... punching its buttons he found that its screen doubled as his car's information and maintenence center.
I think its time to read the owner manual and see what options the car does or does not offer. and go from there.