You cannot correctly abbreviate it as soku without the hisouten part, as 10.5 was known as SWR in western territories.
12.3's titular character, Hisou Tensoku, appears in Sanae's ending, where Suwako explains the meaning behind the name. This also rules out Unthinkable Natural Law, as it was named not as a natural law no one saw coming, but as a being concieved through disregarding natural law.
The only other recent title with no English tagline was 12.5, which is usually refferred to as DS, therefore the only logical abbreviation for 12.3 is HT.
...wait, the whole point of Hisoutensoku is the rejection of common logic...
Fuck.
Let's just call it Donkey Kong 64.
Also, that's not how you eat hotdogs.
No wait, CGNO is the best abbreviation.
But that would also mean 12.8 would be YD.
You Americans fucked up the consistency of the western Touhou community.