I only got 3 yellow crates which contain keycards, and there is a bit of info on the game's wiki that says they may drop from certain enemies, but so far I have not been lucky enough. Which enemies should I farm for keycards? How does the drop chance change when the difficulty level is maxed out?
Keycards can definitely drop from multiple enemies. I am not exactly sure which ones, but I think the Vagrant versions that spawn there, as well as blighted enemies. Maybe even more.
The dropchances are increased the higher the difficulty is. So playing on monsoon and having taken a long time should help the drop rate. Also collecting the 56 Leaf Clover should help you, since it increases item drops from champions.
And regarding your last question: the difficulty never maxes out. The bar can only get so full but the difficulty will continue to rise. It only rises at different speeds depending on the initial difficulty you picked.