score:11
Are there any administrative considerations?
No. As a UK citizen with a UK passport you can enter and leave Austria without any other documents required.
I believe that train to centre is quick
It is! There are actually three different types of trains: CAT (City Airport Train), RailJet and S-Bahn. CAT is dedicated airport transfer which makes it quite expensive (like the Heathrow Express) and that's why it's heavily advertised.
In my opinion the other two are better options: they are regular public transportation, and you can use the same ticket which is half the price of the CAT. Travel time and frequency of departure is roughly the same and you have more options of where to exit or enter in the city.
See https://www.wien.info/en/travel-info/to-and-around/airport-to-center
Upvote:4
As far as I can see, all I need is to ensure my passport is stamped as I leave the airport so then when I return for my Bangkok flight it is clear that I've complied with the Schengen 90 day rule.
Yes, nothing has changed and passports will still have to be stamped.
The planned Entry/Exit System (EES and the corresponding ETIAS registration) has, again, been delayed until (presently) the end of 2023.