I found Presenterm [1] to be optimal for me. Simple and works in the terminal, yet powerful to export to PDF and HTML. It supports Mermaid and images. I'm also collecting a list [2] with other Markdown-first presentation tools, and according to the git stars, reveal.js seems to be the most popular. Tough for me, it was too heavy.
i made a toolset i call "mdslides" for making pure HTML+CSS (no JavaScript) presentations in Markdown. it's just a CSS stylesheet and an 8 line Awk preprocessor for a slide delimiter, adding just enough HTML wrapping to work with the stylesheet. the stylesheet adds page breaks at each slide so you can get a PDF by asking your browser to print/save-as-PDF. it should work with any CommonMark Markdown formatter (i use "md2html" from the MD4C project).
I tried to use pandoc+revealJS, then tried presenterm (which was really nice but didn't give me enough control over font sizes), and then settled on Marp, which worked great.
[1] https://github.com/mfontanini/presenterm
[2] https://www.ssp.sh/brain/markdown-presentations-or-slides/