What to do with old (-er) steam keys

You lose a giveaway slot for every giveaway you delete. If you only have about 25 now... then perhaps it might be best reconsidering your plans.

Steamkeys generally never expire - except for trashy games from cheap bundles (GoGo, DIG and the like). If you acquired them in legal ways you should be good. If in doubt b/c you got them from sources I mentioned, there's a dedicated threat about revokations, so you can do some research before making giveaways.
https://www.steamgifts.com/discussion/Dgscb/a-list-of-games-whose-unredeemed-keys-were-revoked-last-update-17-jan
Personally I have done quite some giveaways and never had a bad key I bought from reliable reseller.

Random keys giveaway?

If you're not confident of the keys and are not bothered by cv, I would use this https://www.steamgifts.com/discussion/hK2kQ/orphan-keys-drop-thread

https://www.reddit.com/r/RandomActsOfGaming/ is a good choice. No first come first serve giveaways and no posting of codes. You choose the winners and if some codes don't work , there is no problem.

You could start a discussion and create a limited whitelist of people (50 to 100) with the warning that they need to accept the deletion of the giveaway if the key doesnt work. So each people that complies on your discussion is on that temporal WL.
Cons: you need to write down the whitelist for later deletion :P