Dotted circle for songs no longer available on iPhone -- How to prevent?

One of my LONG TERM frustrations with Apple is how my phone constantly "loses" music from the music app. This problem seems to focus almost exclusively on music and albums that I have imported from CDs that I purchased back in the past. I have a LOT of that music, and I want it on my phone since I am frequently out of cell coverage in the PNW. It is VERY frustrating than being out in the middle of nowhere, thinking about a song/album that you KNOW you have, and then it is no longer on your phone. The frustration is worse because the problem seems almost entirely focused on music I didn't buy from Apple.

The good news is that I have figured out that I can reinstall the songs by unchecking the "Sync Music" option; applying that, disconnecting from iTunes, restarting the phone, reconnecting to iTunes, and then rechecking the "Sync Music" option. But since I have so much music, it takes over two hours to copy all of it from my computer to my phone.

So back to the question: How do I stop my iPhone from deleting or otherwise making my music unavailable on my phone?