Ah, the superiority of MorphOS!

My (former) favourite Linux E-Mail client Claws-Mail:
Failed several times in getting OAuth2 verification to work despite reading the Wiki thouroughly and watching some youtube tutorials regarding this matter an trying really hard for about 2 hrs.

My favourite MorphOS E-Mail client Iris:
Succeeded at 1st attemp within minutes in adding my GMail account + Oauth2 without reading any documentation just by following the instructions on screen.

Looks like I may need to resort to Thunderbird or some other bloated E-Mail client on Linux after all. Claws is nice, slim & fast but Oauth2 support in it's current state is a mess.