My own location sender app had problems reconnecting BT. Also it slowed down after 20 seconds (could be caused by the slow phone having to send GPS data while maintaining a wireless hotspot).
Luckily I found an app that did exactly what I want called SensoDuino (thanks!). It’s a little old but if you search for an app that sends sensor-data from Android over a bluetooth serial connection, this does the job!
Because it works almost the same as my own app, I only had to change the BT-listener / Rosmessage-sender a little. And it seems stable and fast enough. Hurrah!