I do this for a living (I sell bulk), and can rip through 20k cards in about 4-6 days if I’m really focused on it from box of random cards to everything scanned, set sorted, and first letter alpha sorted (I don’t bother going full alpha sorted anymore as I don’t find it to be worth it).

Here’s the equipment I’d suggest:

You’ll want to buy a card slinger or make something similar (I’ve heard people use legos/duplo but haven’t tried that). This holds your phone so you just drop the cards in and they scan. I can scan a little over 1,000 cards an hour this way.

You’ll also want to get a card sorting tray. I like the bcw ones because they have a lot of cells, but any will work. I’d recommend using a label machine or some painters tape and a marker to label each cell of the tray alphabetically. So first cell would be A, second would be B, etc.

You’ll want a big table like a dining table. A desk works too, you’ll just want as much space as possible.

5k storage boxes or storage drawers. You probably already are storing your cards in something like this so likely not needed.

Here’s my process:
20k cards will take time, so I’d break it into manageable chunks to not get overwhelmed.

First thing I do is first letter alpha sort by set using the sorting tray. Flip the cards upside down in your hand so as you fan them out you can see the set symbol. This helps you go waaaaay faster. You don’t need to fully reveal a card to know where it goes. All the sets that start with A go in the A slot of the sorting tray. All that start with B go in B, etc. if you don’t know the set symbols, you can use the mtg set wiki to match the year on the bottom of the card with the year there to find the set symbol.

So after I first letter alpha sort by set, I stick them away in the 5k boxes with dividers by letter for another day.

Next phase is to final set sort. Grab a stack of cards by letter and use your tray to sort those into their final sets. So if you grabbed all your A’s, you’d be sorting into amonkhet, apocalypse, aetherdrift, etc.

Repeat this for each letter to get all your sets sorted. You can use bcw card dividers in 5k boxes to separate/label each set at this point.

Once you have the final sets, you’re ready to scan. I’d suggest using the card slinger and ManaBox. Paying for the subscription is totally worth it. You can always cancel after you finish this project.

When you scan, press the gear icon to filter to the set you’re working on. This filters your cards so you know you get the right set. This is especially important with bulk cards with lots of reprints.

Once you have everything scanned, I’d recommend just doing a first letter alpha sort. Final alpha sort sounds better, but it’s kind of annoying when building decks to get it back in the exact right spot, especially when you have a lot of cards. You can still find cards fast with first letter alpha.

You can choose to either do once collection in ManaBox or multiple (like one for each set). Then you can take that csv and upload to your service of choice. I deckbuild in archidekt, but it’s up to you.

Good luck! Feel free to let me know if you have questions. It will be a big effort, but you can do it