Sending newsletters in HTML...no problem. Send them scheduled with Simplenews Scheduler and Swiftmailer.
There are some examples.
You can choose HTML
Sending with cron to avoid spam. Tests are sent immediately.
Set your mail system to swiftmailer
Ik you use CSS and images...
Make the links absolute for the images and add your CSS inline.
There is a subscription block.
Plan the letters as you want.
Look at the status of the letters. Content => Newsletter issues