Here are the current border restrictions for the countries on our radar. Not to say production cannot negotiate something around the rules with local authorities, but it's much easier to work with the flow than against it. I'm assuming everyone in the cast and crew are vaccinated with recognised vaccines.
Switzerland and Ireland require no testing or quarantine for vaccinated passengers.
England and Scotland look at travel history in the past 10 days. The US is on the amber list, while Switzerland is on the green list. For those on the TAR plane, they'd need to take a pre-departure test 3 days before travel, and an on-arrival test on or before Day 2 for vaccinated travellers. No quarantine is required. Note that a new set of rules comes into effect on 4 Oct.
Greece requires no testing or quarantine for passengers coming from the US, UK, and Ireland. However, there would be random rapid tests.
Europe is a safe haven for TAR filming; it gets much more complicated once you head east past the Middle East, where almost all leisure travel is blocked. Business visas might be negotiable for these countries, though.