I was using the 3rd one.
Still trying to find a solution but yesterday I found this topic about the RTC being automatically synced every 24h if it's connected to the Arduino Cloud (which it is). In my cloud settings I do have the correct timezone and DST, so if it is indeed the cloud updating the RTC I'm still stumped why it applies the wrong settings...
Maybe that discussion will clear some things up but at the moment I've set the ntpClient to call the server every hour for an update. If for some reason it does relapse it will hopefully correct itself soon after.