Hi Mr Walken!
I think COMC is definitely worthwhile for you. Your best bet would be to separate out the cards that would have a COMC low price of at least, say, $2 or more and send those in first. You might try a small batch with priority processing so that you can get the hang of things sooner. If you have some better cards ($5 to $10 or more on COMC), you might even submit a few with express processing.
How fast they sell depends on a lot of factors. The one you have the most control over is how aggressively you discount the cards. If you're offering hard to find cards of high demand star players at 90% off book, they'll blow out the door pretty fast. If you price them conservatively, you'll probably sell about 2 to 5% of your cards per month.
Cards on COMC seem to sell about as fast as they would on eBay, but the storage fee is a lot less (especially these days) and you don't need to do anything at all once the card sells.
It's true that cards were overproduced in the 1990's, but it's also true that there are a lot of 1990's cards that are very hard to find if you're looking for them. Hard to find inserts of star players seem to do well. Scarcer parallel cards (for example, First Day Issue or Finest Refractors) seem to do well. Rookie cards of star players (Jeter, Griffey Jr, etc) seem to do well, especially if you'll accept offers of 50% off your asking price.
1990's Cards that book for $2 to $20 are hard to sell these days on eBay. They've always been the sort of thing that you would put in your eBay store and hope to maybe sell about half of what you post within 1 year or so. When eBay stores cost 2-3 cents per listing per month, you had the luxury of parking a card and waiting for the right buyer. Now, with fees as much as 50 cents per listing per month, you really need to post stuff that will move (unless you also spend $50 to $300 per month for a premium store).
If you look on the COMC sellers list, you'll see that a lot of the people who have been around for at least 1 year have sold 50%, 80%, even 90% of their cards or more.
Everything will sell on COMC if the price is right. If you ever get sick of the whole thing, you can either get all of your cards returned to you for a $25 fee, or you can go on this board and offer to sell your entire port for a tempting price and someone will buy every card you have. Good luck!