Is there a way to affect the behavior of all vim normal-mode commands?  Say, after every command succeeds, I want to trigger some post-behavior.
Is it possible to affect the behavior of all vim commands in normal mode?
vim
No, you cannot do this; you only have the events at :help autocmd-events at your disposal. You would have to overload every normal mode command with a custom mapping; this is especially tricky for commands like y{motion}.Please give more background; what is your final goal? When a question is only about a small technical step, it's difficult to provide a good answer. If you don't tell us why you want this, it's easy to succumb to the XY problem.