Only 100 undo-levels / 500MB for undo? Any way to increase that further?

There isn't any limit on undo actions. Did you any problem with undo actions ?

I just set undo_depth to 10 as a test, and did a bunch of Move and Delete actions on the Cadence_Demo board file. OrCAD did indeed only let me undo the last 10 actions; Undo is not able to go back far enough to restore the board to its original state.

max_undo_memory
undo_depth

"Specify the maximum memory (in MB) for storing undo history. Default is 20 MegaBytes. Range is 0 to 500."

"Specify the maximum number of commands for Undo history. More undo history takes more memory. Default is 10 commands. Range is 0 to 100 commands. The actual number of commands in the undo buffer is dynamically determined based upon this value and the undo memory consumed to undo each command, the total of which must not exceed max_undo_memory."

I also just found that it doesn't always work on individual actions:
Eagle: Activate Delete command and start clicking on items. Using Undo undoes each individual deletion.
OrCAD: Undo undoes the entire usage of the Delete command. So if I clicked in seven things to delete them, Undo doesn't undo the last individual deletion, it undoes all seven deletions. But with the Move command, Undo works on each individual move. It shows up that way in the Undo-history dropdown: Numerous Move commands, but only one Delete.

Update: Apparently the 500MB/100-actions limits are restricted to the UI only. Why do that??

I manually edited the env file to:

max_undo_memory 5000
undo_depth 1000

PCB Editor seems to be ok with that. The undo history dropdown was three columns wide, and I was able to undo the entirety of about 200 Move commands, restoring the board to its original state.

That issue with Delete is still not good though. I can click Delete and delete 15 items, but Undo undoes all of them, not just the last pick. Maybe there's a setting for that too?