You can use Badgeify, which adds any app icon to the menu bar and shows the number of notifications. You can also utilize any SVG icon as an app icon.
If you prefer a free or open-source app, Doll(https://github.com/xiaogdgenuine/Doll) would be a good choice.
I am the developer of Badgeify, and we offer a free trial license for those who want to try it.
