I definitely like the idea, but I also like the Trill having some kind of distinguishing feature of their own.
Maybe all Trill have their spots but joined Trill could inherit some other feature from the symbiont or perhaps different stripe/spot patterns? Those “extra” spots could still develop more and more over time like you described; that’s a cool idea (even though it would make lying about your age difficult 😆)
If that were the case, then it would be cool to see Adira develop just that set of stripes after they joined with Tal.