[stoppingdown]

Actually it's the way I'm trying now, this app seems to work fine:

https://monitorplus.cc/

I tried apps in the past, but none was good enough (the official one from Sony doesn't even start!). This is a real improvement — with the a6600 unfortunately it doesn't support wireless, but I'm testing a 2m cable apparently without problems. Good solution, if confirmed, since it doesn't require extra stuff in the backpack. As soon as the weather turns better (one week of cold and rain in the forecast) I'll check if my Sony XPeria 10 III is bright enough in the sunlight.
It's well complemented by the JJC RMT-P1BT bluetooth remote that also provides a remote custom function (that I use to activate magnification of the focus assist) and step-by-step manual focusing.