Support apparently read the forum 
and suggested temporarily switching sim cards.
I put in a simcard from Hey! telecom, and I received update FP4.TP33.C.0146 of 05 February 2026 (Android 13), Security patch level: January 2026.
After that, I got update FP4.QREL.15.16.1 of 30 March 2026. Android 15, finally!!!)
Then I put back my own Proximus simcard, and … drumroll… I got FP4.QREL.15.17.2 wich is just released 1 June 2026, security patch May !
All settings, even home screen widgets, were kept as they were.