I saw that SDK 7.1.0 was released (typical to Garmin it's called 7.1.0-RC2-2024-02-14 inside..., though probably it should be called 7.1.0-RC3-2024-04-01)
So I tried it again. This of course means changing all my apps one by one with changes that make them incompatible with SDK 6.4.2. This could be "OK", but nothing works in 7.1.0! I see that none of the things reported by multiple people about 7.x were fixed...