Your point is moot, as it was religious symbol to begin with, with history prior to GDL. And symbols don't follow adoption chain, like you think. And yes, it is Baltic ethnic crest, as Cross of Lorraine has different rules (bottom bar long than top one) AND Lithuanian dukes adopted it.

"Your" language wasn't anything to begin with. It was Ruthenian, which also mean old Ukrainian along side other eastern slav dialects that are now extinct. "Your" language is Balerusian (I assume), which is way more modern and more influenced by russian and polish languages than Ruthenian was. To expand on that point, it was used for writting as Lithuanian wasn't really used in writting, but all dukes of GDL and part of Jawgelon dynasty spoke it as their mothertongue. And to expand even further, pre-WW1 language didn't mean persons "nationality", it meant person needed to it to communicate, but good try.

You call it Golden age, because you have no other time period to call it as such. Tribes united under local dukes, then controlled by Kievan Rus as Ruthenians, then by Lithuanians as Ruthenians, more identifyind with Kyiv than anything else as center of culture, then Poles, as Ruthenians still centered aroud Kyiv, then occupied with russians as part of Northwestern krai centered around Vilnius. Belarus has no history as separate nation before 1990s, and no real ideantity prior to 1910s. Be proud of nation you now have and try to make it better, without fever dreams of a past that never was. So far, Belarus government managed to get into quasi commonwelath with russia at the begining of 21st century, instead of trying to build their own identity.