Don't forget the Saxons (Germans for those who would not know; I know you know this @Blebea Cezar-Iulian )!
In 1991 there were just 100 000, representing 1,2 of the population of Transylvania. Also, the Romanianization policies of the Ceaușescu led to Romanians comprising 75% of the population and Hungarians being concentrated in the Harghita, Covasna and Mureș counties, so instead of a peaceful Belgium-like state, I predict an Ireland-like tense political situation.
The longer that the debate continues over the fate of Pomerania and Silesia, the more likely it is to remain under Polish control.
Frey gained power on the promise of making Germany great again and he would not simply abandon the annexation of the Eastern Provinces because it takes too long. Poland also regained their 1938 Eastern Borders, so why expand into hostile Ukrainian and Bielorussian territory, if they won't recolonize it with Poles?