Hard agree on the room for optimising. I won't dispute that, even as someone who thinks parses etc are dumb. (I see skill expression more of executing mechanics consistently and adjusting your rotation accordingly to maintain uptime rather than chasing perfect numbers through greed and overgearing)
As someone who likes support though, I don't think GCD cards is a good idea from a fun or rotational standpoints.
Fun wise, there's a flat number of gcds per minute. By turning the cards gcd, this limits what options you have for spells which is the issue with all healers- lack of damage spells. Astrologian would still end up having a very dull rotation of Draw-Navigate to target-Play card-malefic-malefic etc since all your stacks will be detonated by malefic as you said.
Or if they are detonated by other players...your still spamming malefic when not using cards since someone's nicked your stacks
On rotation- This would also have the problem on controller of extreme APM scrolling through the party list depending on what card you draw no matter if its teh current invisible cards, or the previous fun cards. Not everyone will have the idea or hotbar space for a macro to every player. Assuming your rotation involves more cards than before that is to set up the various buffs.
Too few cards and you're stuck casting malefic, too many and you easily get a repeat of 5.0-5.3 Ast issues.