After trying a few also we ended up with EmailOctopus because of simplicity (we only send plain text emails) and cost. The trick was using their Connect [1] plans so it could send via our AWS account, which is cheaper (we pay $30/mo for the 10,0000 subscriber plan).
I also tried Loops and wanted to love it since they're perfect for SaaS companies, but back when I tried them we just got a ton of spam subscribers since they didn't have any built-in mitigation, so our list (and cost) grew.
But that was in their very early days, so I assume they've resolved it by now and I'd like to try them again at some point since they're much more modern and purpose-built for SaaS (and a YC company).
[1] https://help.emailoctopus.com/article/161-what-is-emailoctop...