Cd storage / backup

I used a Mac computer to rip all my CD's to FLAC using xAct audio software.  I use a Google wireless network (3 piece) which employs a mesh network between its devices.  This is much better than traditional routers and extenders.
I copy all Flac files to NAS devices (wired) from my Mac.  From here I have another mac in my media room that runs Roon.  Roon does a great job of creating a library , getting content related to your media and presenting it.  

i have several systems running Roon compatible devices.  For critical listening I use an Auralic Aries and OPPO UDP-205 for lossless  and HI-RES playback.  For whole house and party mode I have several Sonos devices which also work with Roon.  The Roon software itself runs on any computer, tablet or smart phone.    

Bluesound vault II works very well.

Whatever you choose as a storage device have a second device of the same capacity as a backup. I helped manage personal computer systems for a large publishing company and part of that job was doing grief counseling for people who never backed up their work. It's not a question of whether a hard drive will fail, it's when it will fail. It will fail eventually.

My digital library is about the same size as yours would be, with the addition of a couple of thousand photographic images, and I have three backups. One is kept in a waterproof, fireproof safe that itself is inside a locked metal cabinet designed as a gun safe. Another is kept at my daughter's house. With an inexpensive program that does incremental backups, keeping current with the backups is a painless process.

You’ve gotten some good suggestions already. I could add to it as I have over 4000 CDs (in addition to the 20 Terabytes of flac/shn files stored on external drives). You can kill two birds with one stone, depending on how your CDs are currently stored. Most people use/need/want the fancy jewel case any store bought commercial CD would come in. If you sell your CDs as you plan terrific for you, but in all likelihood you wont find a buyer for all of them, unless your price is such that you’re practically giving them away. So.... throw out the jewel case and store them in 100 pack spindles, like this https://www.amazon.com/EMPTY-CAKE-SPINDLE-100-DISCS/dp/B0089Y6WPG Then, after ripping, retain them as "backup" to whatever hard drive you rip them to. Of course, store as a flac or shn, not wav. 500 CDs takes very little space, as in less than 400 Gig. Practically any external drive would suffice. Best of luck.

EDIT:

If accurate ripping is of utmost concern, you want to use this http://www.exactaudiocopy.de/

+1 gdhal, I've got several boxes of my ripped CDs in my basement. I think I'll stack them on spindles, retaining the liner notes, and toss the jewel cases which are a giant PITA anyways. To add to that, reselling the CDs while retaining the ripped copy is unethical at best ( the artist does not get paid for his intellectual property) and technically illegal.  Ripping them to a hard drive is perfectly OK under the fair use doctrine which allows you to make copies for your own personal, non-commercial use but not to sell the original.