Should I send marketing email from a subdomain?

Usually yes, because it separates the reputation of your bulk mail from the mail that has to arrive:

• Reputation is per-domain - a spam complaint spike on a campaign can affect your ordinary business mail.
• A subdomain isolates it - news.example.com.au carries its own sending reputation.
• It needs its own setup - SPF, DKIM and DMARC alignment for the subdomain, not just the parent.

Check the DMARC sp tag too: it sets the policy for subdomains and is easy to leave weaker than the parent.