@”Kapitán Denis” well, I use Hungarian (national) and Magyar (ethnic) when speaking/writing English, of course it doesn’t happen too often. However, as Shaokang said, I often say for myself that I’m Serbian, but never a Serb.
Funny, when speaking Serbian I always correct people when they call me Serbian (Srbijanac), just call me anything but that. For us, in the north, Srbijanci are south of Sava and Danube, it’s a regional and cultural term which of course has certain stereotypes attached to it.
on the Czechia topic, Czechia sounds weird to me, but is faster to type and more similar to Češka/Česko which I would normally use.