Okay, you knew this question was coming - surprisingly, I can only find piecemail tips and tricks in various threads, but not one cohesive tutorial, so alas, I return to here once more. With gamefonts!
It did work as I expected it to work:
Use RevolutionDCM's GameFont files, which luckily are already expanded a lot to have enough room to accomodate for more religions.
Tell ReligionInfos.xml the index (starting at 0) of the icons within the GameFont files.
And thus, rearrange the religious icons in the GameFont files to order them to your desire:
View attachment 644815
Then (a step I haven't done yet, because I first wanted to test if this would actually work), create new icons and fill in the blanks.
This works, in so far as that the logic I describe above seems to apply:
View attachment 644816
This does not work, in so far as that I now have white circles with black numbers floating around my screen for no discernible reason (see the tech bar, see the great people bar, see the Workshop's tooltip, see the Worker's movement):
View attachment 644819
The only thing I can think of, is that the default 'white circle with black number' includes a special marking, that tells the game 'look, there is an icon here', while me copying icons away replaces their former space with 'nothing', which makes the game interpret things incorrectly. If so - how would I functionally go about rearranging icons and adding new icons? Would I need to copy-paste every icon to the three rows above (with yields, commerce, and war/peace icons), because there the game seemingly handles completely white icons correctly, and so I can use that space to swap around my icons? Or can I just insert this theorised 'special marking' on the now-empty white icons? Or...?