It’s good to hear that you’ve given Manjaro a try and that you give it 9/10 stars.
With regard to Grub Customizer, there have been many (probably hundreds) of forum threads over the years discussing this app, and it turns out that it is known to cause more problems than it is worth – at least when using it with Manjaro. Here is a comment from a couple weeks ago that gives a few links that you can check out: Unable to change grub menu entries because of grub customizer not working - #6 by Ben . Bottom line: it is best to avoid Grub Customizer, as it will save you from headaches down the road.