The table format of the native markdown web part is for your reference:
As @Trevor Seward said, you need to develop your custom markdown SPFX.
My test MarkDown SPFX for your reference:
You could get it here:https://github.com/Amos-MSFT/SharePoint-FrameWork-Demos/blob/master/ReactMarkDownSPFX/src/webparts/reactEmptySpfx/components/ReactEmptySpfx.tsx
I use react-markdown-editor-lite in my test project, Github address:https://github.com/HarryChen0506/react-markdown-editor-lite/blob/master/README.md
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