Will they potentially seek union with Danubia?
No. Officially they're the 'Protectorate of Szeklerland', with the Habsburg Emperor as head of state and Danubia responsible for foreign affairs and defence. In practice, on the foreign affairs question it means that Szeklerland sends a couple of officials to be part of the major Danubian embassies overseas. Defence is out of local hands because it is subject to articles of the Treaty of Pressburg, which limits the size and armament of their armed forces. Other than that they use the Danubian currency (the euro) but are fully independent in domestic affairs, although there is a certain kind of very high level 'oversight' from Vienna.
And what is their development like as they're an "enclave nation"?
So, as background, Romania as a whole before this was more or less at the level of development of an OTL Southern European country of the same period (e.g. Italy). Szeklerland itself obviously struggles a bit as an exclave but there are agreements allowing its citizens to get easy work visas in Romania and allowing free movement with Danubia. So it's doing pretty well from remittances. It's still poorer than Romania but has enjoyed growth as a kind of 'off-shore' centre for Romanian and Danubian business.