But life is too short to not use a slides tool. Manually aligning bullet points, moving around ascii art, all of the settings to remove the vim status bar and a custom syntax mode to style text… I don’t know what you gain other than oh-that’s-not-google-slides brownie points. Your time is probably better spent working on the presentation.
Well, there's at least better version control with git, and the other general benefits of plain text. If you have a bunch of presentations, easier to search among them based on content if they're plain text.
Though, presentations being presentations, the fact that they generally lack content to be useful on their own without their presenter, makes them kind of disposable as documents.
For diagrams, I'm using draw.io (it seems that it's called diagrams.net nowadays), it's FOSS, easy to use and feature-full. It's then easy to add it in the presentation by using the markdown syntax for images.