I was advised to 'just resolder some of the components around the PSU'
and that's what I did. Is it possible that my resoldering may have
allowed an already broken component leg (like on a voltage reg) to
fall away / disconnect further?

Try checking and resoldering, if necessary, parts of the vertical deflection,
horizontal deflection, and syscon.

It sounds like the techs might have indicated other parts besides the power
supply. - Reinhart