Custom Latex package

I tried searching but could not find.

For some embedded latex in markdown, custom latex packages might be needed. Is there a way, I could intimate that also within the embedded tags here, or via admin panel?

For example, to use merged rows in table, markdown does not support that, and I guess html elements cannot also be used, so I thought of using latex table via nicematrix package as hinted here

A less than ideal way would be to create in excel and paste the image, but source is lost (or excel to be attached always nearby). But if latex, then its source in editor, and table as output, so we can easily edit.

Kindly suggest.

WIthout looking deeply into your exact issue Discourse Math Plugin may not be exactly what you want, but it’s the supported solution.