The Kodak Retina IIa has a pretty bad finder, tiny and no frame lines. No offense Tim, but OP clearly stated he considered a IIIC mainly because of the finder.
IMHO, the big C finder is a little better than most of the later Japanese compact rangefinders, but not much. I've never looked through a small c finder, but a IIa I have. No comparison.
I really like my IIIC. Lens is excellent, finder works very well with glasses. But, OP, the meters on Retinas, even when they still work, are certainly not to be trusted for slide film. The ergonomics are questionable - bottom wind, little to grip on the right side means that one-handed shooting is impossible. The hood cannot stay on when folding it. They all need a service today.
It's very tall, not that much smaller than your Yashica. If that is reliable and you get along with it, I recommend you stick with that. I'd also prefer the slightly wider lens for my only camera. Somewhere in the depth of this forum there's an old thread with Electro 35 pictures from western China which really impressed me. The lens appears to be excellent.
If manual exposure is a priority, there are many alternatives, some of which have been named in the other thread you opened, like Minoltas, Konicas, Canonets, Olympus. I'd go for more modern ergonomics.