Speed mix up:
I'd go for A:6 or 33B, both go in the unblockable window, which is good because 33B is unsafe.
High/Low mix up(also varying in speed):
Ok first: If they reGI with the right timing but in the wrong zone(high/low) they get Counter Hit!
That's the only thing that makes Post GI High/Low mix up half useful.
I'd use:
2KK, it's no Natural combo but the second attack is quite safe -9 or something. It's also a Natural Counter Combo.
Gives no advantage though. You can also use 1K but the damage is minimal and it only gives advantage on CH.
22K, it gives nice Tech Traps on CH (WR B catches all tech i think)and is Counter Hit Checkable.
KK(NC) and KKK(NCC) both give advantage and KKK is safe here, when the first two Ks hit the third can't be crouched.
4K gives huge advantage.
33A, it is Taki's fastest PO shift. It opens up nice mix ups. 33ABBB is also a NCC.
All of the above goes in the unblockable post GI window so they have to GI that when you do it fast enough.
Whiff GI punishment:
Bomb is IMO better than 22B because bomb punishes whiff GI too, yields more damage and is safe.
General thoughts:
The problem with taki is the damage. Because of how little damage she does you generally can't take as much risk as with other chars like Asty because one ill-timed GI can cost you so much health, you may have to poke your opponent 20 times to make up for it.
I'd use GI very sparely. Punishment is IMO way more important with Taki and her biggest strength.
Anyway, noone mentioned throws as post GI. It's usually not such a good option but because Taki's throws are relatively strong, compared to her normal attacks, i think that makes them also quite valuable as post GI.
And one last thing some people may not know:
Post GI throws can't be crouched! That kinda sh** only happens online!