How about Charles Dance?
A bit old perhaps (how old should Thrawn be during the Mandalorian?), but a few decades ago he definitely had the right face for the character.
I think there’s a decent amount of leeway with the face overall though, and I think the eyes and especially the voice are the two things that’s really important when it comes to casting Thrawn. Not counting the audiobook voice-actor that imitates Mikkelsen’s performance, the only two Thrawn voices that has worked for me is Lars Mikkelsen and the voice-actor from the audiobook version of Outbound Flight (couldn’t find his name). Both have very calm and measured performances. Other attempts have seemed to stereotypically British or just generic tough-sounding bad guy.