Luxembourg

Brussels, where you may well fly into, has direct trains to Luxembourg City.

How many days will you give Luxembourg? Do you want to visit the countryside or just Luxembourg City?

Or is it a dip your toe in to say you have been to a small country?

Luxembourg is 3 hrs by rail from Brussels. It's a slow train and stops at many smaller stations. But it's the best solution there is, unless you want to drive, which will be faster. It's fairly isolated, which is why their little pocket of culture continues to exist intact. The only other option is to reach it from Trier (Germany), but Trier is not that easy to get to either. It is at the end of the Mosel River, lined with vineyards, and very picturesque, so there are tourists, and it is worth seeing in its own right (Roman ruins).

Hm, what about the airport? Could you fly into Luxembourg (open jaw) and leave from Amsterdam? I don't know the RS tour myself, does that fit in for you?

Lavandula

This summer's trip I'll be back in France , also staying in Metz, most likely if I don't choose another place in Lorraine (also likely) but the point here is to do a day trip to Lux. City and explore that place. Very convenient from Metz.

But it's the best solution there is, unless you want to drive, which will be faster.

Why would that (= train) be the best solution? Many tourists book their trip to Luxembourg (and Dinant) from Brussels with Brussels City Tours (= bus daytrip) https://www.brussels-city-tours.com/luxemburg-and-dinant-the-beauty-of-the-ardennes

You can find the very positive reviews at Viator, a third-party reseller of this same trip.
https://www.viator.com/tours/Brussels/Luxembourg-Day-Trip-from-Brussels-Two-Countries-in-One-Day/d458-2200DEALB