Looks very decent, I might switch to this for writing my blog! :)
I was working on a project called karasu that does some of the things you are talking about, it renders everything using a real LaTeX compiler so all the fonts and TeX packages can be used, though I mainly use it to discuss math with other people to supplement email.
The rendered images are cached so it somehow compiles faster than using pdflatex/pdfxetex directly.
How does it compare to Overleaf?