I've used it for years, no issues with delivery to gmail or outlook.com / o365. They've got a bit about their delivery system mentioned here: https://mxroute.com/docs/#/Presales/ipreputation

It's pretty basic in terms of webmail/core functionality but it receives and delivers mail well from the various IMAP clients I use, which is all I need.

Really evident if you zoom in a bit and look at March/April 2020, v6 usage stops dipping as hard during weekdays.

Correct. You can receive emails on any IP, sending is the only time reputation comes into play.

Yep, you just configure your outbound SMTP server to be the SES credentials and adjust your SPF/DKIM records.