I'm happy to report that I successfully changed CSC from XAR to DBT.
Used the FT55 ATJ1 Combination firmware on DUA3 (18.104.22.168), changed CSC and then flashed CTJ5 stock firmware. Got the DUA3 OTA after that, now waiting for the latest update which is supposed to enable ECG+BP in the EU.
Although, I did use the latest tpk pusher and have the latest SHM app (which contains both the ECG and BP inside) on my GWA2 now, opening either ECG or BP on the watch and clicking on "download app on phone" still tells me "this is not available on your region".
So, the update is clearly not live yet in Germany at least.. SHM is still not available. The wait continues lol.
Just to confirm that you flashed CTJ5 Stock firmware just because you did not have the stock DUA3 Files and you took the DUA3 OTA via the Galaxy Wear app.
So if I get hold of the stock DUA3 firmware files I should be able to jump straight to it from the Combination ATJ1 after changing CSC.
@adfree thank you so much for your inputs here and on other threads and also for posting the combination files! First time getting into this Tizen wearable mess and it would have been much harder without your inputs!