A sensible minimum might be CommonMark. This covers the basis that is used in many different flavors. For instance here’s Github-flavored Markdown spec, that highlights where extensions were made.
In terms of extensions I would look at what some popular apps support with which you want to maintain copy/paste compatibility. Don’t know if Discourse is a good example here, though, as it comes with a lot of customizations. The image scaling thing is nice, but I haven’t found it elsewhere yet.
A compatibility with a bunch of code forges might be nice, but on the other hand Redaktor is not really targeted to a technical audience (just guessing). Furthermore code forges also vary widely in extension support. Like Gitea with their Go parser supports a lot of stuff, like Mermaid diagrams and - I believe - table format extensions.
You might look at Lemmy and SocialHome and such, and seek compatibility across existing fedi apps, if you don’t have that already (seem likely with the above implemented).
Table and Image extensions for a CMS-like app (sorry, I forgot the acronym we came up with… was it Federated Information Management?, … no) seem good nice-to-have extensions (e.g. table with colspan / rowspan).