I highly recommend https://knak.com/ - they integrate with Marketo (for Enterprise you can have a direct connection and be able to spin up email sends or programs straight from Knak). It reduced our development time (from 1-2 hours to about 15 mins per email) and our error/rendering issues to 0%. Check them out if you're looking for a user-friendly option to create beautiful emails at scale.
Hopefully this is within community guidelines as you've asked explicitly for recommendations, but I'm sure someone will tell me if not.
But, I work for TAPCXM who have expertise in this area and I have created these templates for clients before. We're a consultancy by nature so if you would like to have a chat to unburden you with these templates, then feel free to message me on here or reach out via the website for a demo of what we can do and how we can help 🙂