If I were to reimagine the Trill, I'd make them naturally unspotted...
The symbiont would be, and the Trill's spots would only present when the two joined. As the symbiont grew older, their spots would increase in number, so later hosts like Jadzia would have more spots than earlier hosts. Like rings in a tree.
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.
For the baseline Trill or the joined ones? If the latter, I'd really hate to put Adira through that. They're finally healing and part of a family unit; would break my heart to have them go through a "what's wrong with my forehead?!" arc in the final season. lol
Edit: Thanks for the picture. I completely forgot Jadzia had that in the early episodes.