I don’t know whether it helps at all but I am sending a quasi-daily datestamp text message from another phone to my Librem 5 - and I have experienced no loss of text messages (apart from when the tower went out for X days during the flood LOL). The reason I am doing this is that I don’t receive enough text messages in real life but I still want to monitor for possible regressions or breakages, such as is being discussed here.
The sending phone is an iPhone, on a different carrier - so should be very vanilla. The sending iPhone has been ‘trained’ that the receiving phone is no longer an iPhone (otherwise the damn stupid thing will try to send via iMessage i.e. via Apple, and there will be problems).
Receipt is essentially instantaneous, typically. The text message makes a net journey of 1 metre but via a very circuitous route.
This is awkward to keep track of. Lots of updates are coming through. Today’s test was after I applied the latest updates (which updates must have been released overnight). Text messages worked yesterday too.
I am using amber. Keeping it simple.