You could write an AutoHotkey script triggered by keyboard shortcuts to surround a selected word or words with markdown symbols.
There is a thread dedicated to creating such macros. I bet the folks who have contributed scripts would enjoy noodling this with you. Please share your AutoHotKey macros for Scrivener - Scrivener / Scrivener for Windows - Literature & Latte Forums
Update: And of course, I didn’t read the full thread first. I see that @AmberV has offered deep insights into the “why” of this approach while also providing a link to a script.