The theme directory has a more recent rule that themes are not allowed to inject elements into the customizer via JS. This was primarily meant for stopping people from injecting upsell links in odd places and breaking the customizer UI.
Your welcome screen customizer JS definitely breaks that rule by injecting a linked number. Personally, I feel like this is minor enough to not be an issue in this particular case. However, the TRT might disagree. So, this is more of a heads up. You can always bring it up with the team to see how they feel about allowing it.
The proposed alternative is to use the Customizer API to add such an item, which you already know how to do. I've built something for this primarily meant to be used for upsell links: https://github.com/justintadlock/trt-customizer-pro I just feel like it's a bit overkill for a tiny number.