Best (fastest) way to download large audio CD library to large digital storage device

Google up "CD ripping service". There are lots. Some send back DVDs, some portable hard drives (from which you can transfer to whatever), there's one that even streams to Sonos equipment.

If they transfer your files back to you on a high-capacity disk, you probably would not want to simply run that as your everyday source - it could crash. Search up a good NAS (Network Attached Storage (sorta a big disk on your network without it being a "server" but sorta is anyway) that uses RAID 0 (Redundant Array of Inexpensive Disks - this lets drives mirror each other so if one dies, you're still in business). Synology makes good ones, and small home versions aren't too expensive (versus Corporate-level gizmos).

A good digital music library will 1) be well organized (a gigantic single-level directory is no good), 2) well tagged (titles, artists, year, etc., internal to each file), 3) include album art, and 4) be lossless or high-sample rate.

Ripping CDs is not hard to do on your own. Yes, there is a hump to get over. And it's somewhat time consuming (but far far easier than converting LPs). Lots of good, open source software is available to do the job, so you only really have to buy storage hardware.

Having your whole music library digitized lets you do more than "anything you want" with it, you can do everything you want to - everywhere, all the time. It's terrific.

I'd just find the FLACs I wanted off Pirate Bay and pull then down. I already own the CDs, so why do I need to digitize what I already own if someone else is sharing?

If you happened to have bought those CDs from Amazon, Amazon will "give" you the files on their cloud.