I've found advice from seat61.com useful a few times.

He recommends https://raileurope.com/ -- When I used them back when they were loco2, I was impressed by the customer service. I needed to change my train ticket, so emailed them and they sorted it all out for me with minimum fuss, emailing me replacements. At the time it was a lot easier than dealing with the local railway companies in countries where I didn't speak the language. I don't know if they would be as good now that they aren't a startup though.

History of loco2/raileurope: https://www.seat61.com/websites/who-are-raileurope.htm