I agree with all your suggestions, but want to expand on a couple points: 1) The shared IP pool at cheaper providers like Sendgrid, SES, and MailGun will have lower reputation versus a platform with a guarded (via cost, but also on-boarding mechanisms) platforms like MailChimp, simply due to customer pool and quality of emails sent. I have been hearing from my (BigMailer.io) customers about having deliverability issues with platforms like SendInBlue and MailGun frequently in the last year or so. 2) If the bulk sending practices are healthy and engagement is good, then it's better to use the same sender/domain for both bulk and transactional email.

I would recommend to NOT send different email types from different providers, like use SendGrid or SES for transactional emails and then some other platform for marketing emails, especially using the same sender domain. That's because email header signature and tracking links will be different and email box providers like Gmail can find it suspicious and send emails to Spam. And then there is the issue of processing and syncing bounces/unsubscribes/complaints data.

Keeping all email campaign types in a single platform allows for centralized processing of bounces/unsubscribes/complaints date and protects future campaign engagement + sender reputation.