I have searched and tested several editors during these days, and I was able to narrow down the selection to my needs. Typora and Mark Text.
tl;dr
Typora is beautiful, easy and just works. currently at Version 0.99.32.1 free, when 1.0 is here, you need to buy a license.
Mark Text is beautiful, easy, has a better support for mermaid and IS FREE.
the only downside for me: on my ubuntu it has a slight lag.
Typora - typora.io | github
Markdown WYSIWYG
MD Code Editor
MarkText - marktext.app | github
applause