Well if you happen to find a good app that does what you want on desktop, then you can complement it on Android with Markor , it’s “markdown native” and it has nice extra options like adding list elements, quick formatting etc. All based on markdown, can be easily switched between editor mode and preview(markdown rendered) mode.
Other people already mentioned Roundcube and Snappymail, which are good options already.
There’s also Cypht if you want a different approach. It combines multiple accounts into same interface so you can have a unified inbox.