I understand that dropping the ¨de¨ is something done sometimes in spoken Spanish in Mexico and elsewhere, but it would it be marked incorrect in written Spanish? It seems like there should be some discussion about how this differs from Castilian rules. ChatGPT says re Dropping the de
You may hear:
antes que
or
después que
especially in parts of Latin America.
For example:
Antes que llegues...
Después que terminó...
These forms occur in everyday speech in countries such as Mexico, parts of Central America, the Caribbean, Argentina, Uruguay, and elsewhere, though how common they are varies by region and speaker.
However:
They are considered regional rather than the neutral standard.
In formal writing, education, and when speaking to people from across the Spanish-speaking world, antes de que and después de que are the safer, more widely accepted choices.
If your goal is broadly natural, educated Spanish (which I know you've been working toward), I would recommend keeping the de.
