At 66,000 messages/week, you’d actually be spending around $50-$80 monthly with SES and a VPS + database for handling bounces and unsubscribe requests.

... right, and then I'm responsible for maintaining that infrastructure, and I have to think about email deliverability as well.

Plus Substack discovery does carry some value - about 65% of my new subscribers come via the various Substack mechanisms, the rest are from my own site or "direct" (which I think means people landed on my substack directly and then chose to subscribe.)