I am too lazy to search for all the various threads that deal with ratings.
The basic story is:
The only id3 field for rating is POPM.
And it is an ID3 tag field - other tag formats may have other fields for that.
The underlying format for POPM is always, as stated in the documentation
This description is for only ID3v2 tags. The playcounter is an optional value. The Rating is an integer between 1 (worst) and 255 (best). Syntax: Email|Rating|Playcounter
The email part may vary e.g. for WMP it is Windows Media Player 9 Series and the Media player then interprets the following value , e.g. 128 as 3 stars.
Similar things happen for the other translated ratings, e.g. Rating MM.
The complete string could look like that: Windows Media Player 9 Series|128 - and you see that if you switch on the option to always display the (raw) values in POPULARIMETER and not the translated ones, which would be the off-setting.
If you only write the value e.g. |128| then each player, as long as it reads the rating at all, has the task to translate the rating into its own way of showing it.
A problem that has been discussed many times in this forum: does the player read the field at all? At least iTunes does not. And so it boils down to trial and error to find out whether the preferred player interprets data in that field or not.
You seem to have found a way for your player - even though that uses a user-defined and not even a standard field - but that is probably as good as it will get.
Besides the lack of using the standard field for rating, even more players have problems with user-defined fields.
It would be interesting, though, to see whether your player can cope with POPM (or POPULARIMETER).