Interesting that you mentioned thinness of string. I also have notice such in 2 TOS releases. Namely the Gulda Mozart 20&21, and the Mussorksky Picture at an exhibition.
I found the Gulda Mozart 20 & 21 piano concerto one of my favorite TOS issues. I don't hear thin or strident strings. I'm not disagreeing with what you hear. I find several DG digital recordings from the 70s onward have thin string sound but the TOS are not digital recordings.
Fwiw, looks like Amazon has copies of the Gulda Mozart 25/27 but not at the original price.