With another device I’ve been successful after one night.
I follwed Hylkes manual Migrating OTAA Devices from V2 to V3 - V2 to V3 Upgrade / Migrating Devices to V3 - The Things Network and didn’t delete the device but changed the App-Key. So I’ve been able to see what is happening. The LHT didn’t react to the Downlink which should initiate the reboot. So it took 10 hours and 40 Downlinks until it rebooted and joined V3 without any problems. Firmware on my LHTs is 1.3
I’ll try that with a few other devices and update the first post.