Has anyone updated Ubuntu Touch from version 16.04 to 20.04? Is the problem with the Wlan also present there?

Wifi reconnects much more reliably in 20.04.

In addition, the phone now hangs when booting sometimes at the Sony logo and sometimes at the Ubutu touch start page. It takes sometimes 4 attempts until it boots up.

The startup problems have been resolved with the update OTA-25 v2.
Has anyone updated Ubuntu Touch from version 16.04 to 20.04? Is the problem with the Wlan also present there?

After a long time I find the time to write again. How is the problem with you?
Since the upgrade at the beginning of April to OTA-25 I have the feeling with my Sony Xperia X (F5121), the problem with the wifi module has become even worse. Now I have to restart the phone even more often during the day. In addition, the phone now hangs when booting sometimes at the Sony logo and sometimes at the Ubutu touch start page. It takes sometimes 4 attempts until it boots up.

I have an F5122 and in my case, the loss of Wifi connectivity has no definite pattern that I can see: Sometimes it would reconnect when getting back in range and sometimes not, apparently not dependent on the duration I was connected or out of range. When wifi does not reconnect within seconds, rebooting is the only way to fix it (and it works 99% of time).

Much less often (every couple of months, perhaps), I experience a loss of cell connectivity : the phone would suddenly declare it has no sim card, requiring a reboot to recover. Although infrequent, this later issue can indeed lead to missed calls, as it is entirely up to you to realize you are out of reach (in my case it can last hours...). It's definitely not an electrical problem but really a software problem which was already present when I was running SailfishOS on that phone and that is linked to my carrier using a form of "roaming" with another carrier where it lacks covering. Besides this rare issue, calling and receiving works fine for me and those who call me.

However, unlike in you report, my connectivity issues do not prevent launching the applications I use (camera, browser, email...). But of course, apps needing connectivity do not function properly. For me, commands entered in the terminal perform as expected.

Finally, I'm glad you report here the rebooting issue because I have had this issue too for a few months, after an update (on the candidate channel). When asking for a shutdown or a reboot (most of the time for restoring wifi connectivity) the screen goes black and the phone freezes in that state (power button is inactive unless volume up id pressed simultaneously). However, if you wait long enough (7-8 min?) the phone eventually reboots. I guess some process does not honour the termination request and the kernel waits a long time before forcefully terminating it.