This is the post you want and need to read → Tutorial: 2020 North America 2.20 Map Installation (Stand-Alone without Reformat) with Updated NAV_Voice 06/21/2021
Even though it’s a manual method, that’s what Syn3updater does, so the requirements and instructions are the same.
You will find all your answers about “maps only” update there.
If you ask me, having the chance of just reformatting, I would not perform a partial update since it take a lot of time.