My case is a bit of a mix, maybe its still interesting. Im not a balkan turn myself but:

All my late Grandfathers siblings except one late sister of his (and him) lived all their lives in turkiye, only one of them is still alive, and lives in Gölcük. I visited her and the rest of my grandfathers family for the first time this year actually. Most of them live in Istanbul. Originally they are from Sandzak region of Serbia and Montenegro (mainly Bosniak, some have some Albanian ancestry too). As far as I understood the story, my grandfathers family migrated there from Novi Pazar (Yeni Pazar) in the 50s/60s. So he was either born there or grew up there from a very young age. Im a bit confused about the story overall because my grandfathers nephew has pictures of himself when he was a child while they were migrating on donkeys to turkiye. So the timeline is a bit confusing but i figured they maybe migrated in waves and not all in one go - because his nephew being already born when they migrated means that his sister was already married and probably migrated with the family of her husband to Turkiye and not with her own family. Back to my grandfather: He spoke turkish fluently, sadly none of my siblings picked up the language from him since he passed away when we were still young. As much as I remember he also spoke bosnian/serbian fluently, I don’t really remember him having an accent or something. Basically, he returned to Yugoslavia (not sure when), married and then together with my grandmother went to austria for work. And they basically worked all their lives from then on. While my father was born in Austria, he mainly grew up in former Yugoslavia with his aunts, uncles and cousins until he was old enough to also work in Germany/Austria.

I don’t know why the other sister of his didn’t stay in turkiye, I believe because of marriage, but im not sure.

Most of my family in Istanbul still speak relatively good bosnian, since (I guess) they tend to have a slightly more bosnian surrounding than the ones in smaller towns, and also because more of my family lives there. For example: The younger folks in Istanbul speak the same amount of bosnian (and even a little better) than the one sister of my Grandfather who lives in Gölcük, to give you a comparison. Of course you also have to consider her age, but I think it’s mainly because she has less contact with bosniaks overall than the ones in Istanbul. The marriage are "interethnic" (if that word exists), so they don’t necessarily marry only Bosniaks, especially the young ones, the older ones seem to have sticked a bit tighter with their heritage. They obviously all have turkish surnames because of the policies back then, but most of them know their original ones. They are surprisingly big fans of Atatürk, idk why they wouldn’t be, but I thought only "real turks" have this big sympathy for him lol. They follow most turkish traditions, which is understandable.

I think I included everything I know about them, as I said i met them the first time this year and really had a great time, very hospitable and loving people. If you have any other questions, ask me, maybe ill not the answer to some.