@Stefan_NE Good to hear you finally succeded in having a reliable network.
News from my side:
After 5 days of operation it seems Node_2 is back in a mode of relaible communication, no more issues also with Node_1. These both are the ones sending a lot of data and use HW-serial + "triple-headed" message initialisation.
BUT: Node_3 (BME280) is no longer continously present now, and also Node_4 (sw-serial + "single-headed") seems to have communication problems (didn't yet investigate in depth). So next step will be to first change these sketches also for the use of the triple initialisation. If that works, I'll report - it then may be a good idea to change the defaults in the MySensors-lib (to be discussed).