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.
[1] https://github.com/mfontanini/presenterm
[2] https://www.ssp.sh/brain/markdown-presentations-or-slides/