Here's another classical selection, Beethoven's duo "with two eyeglasses obbligato" for viola and cello. About 9 minutes long.

If you're wondering about the title, Beethoven wrote the piece for a cellist friend who was very nearsighted. Beethoven most likely intended to play the viola part himself -- before he became famous as a pianist, he started his music career as an orchestral violist.

There aren't as many close-ups here. But the viola left hand technique in this video is especially worth watching because Cynthia Phelps plays a monster 17-1/4" viola. Even though she stands 6 feet tall, she has to make the same kinds of adjustments as people who play average-sized violas with smaller hands.

I suspect it's because Ms. Phelps (famously) plays a very large viola, and the longer string may call for a heavier bow stroke. But even on a more typical sized viola, I think the pinky doesn't need to do nearly as much counterbalancing as it would on a violin.

The way I've been going about finding these videos is by trying to think of pieces that especially fit your criteria, then searching for good videos of those pieces that show individual players' technique clearly. There isn't much for unaccompanied solo string instruments other than Bach (at least not until the modern era), so I've been thinking about shorter pieces for 2-4 string instruments that don't have an overly dense texture.

Here are a few more ideas, as links rather than embedded so as not to flood the forum. Given a week or so, I can probably generate a fairly long YouTube playlist, especially if I include single movements of larger pieces.

Schubert, String Trio Movement in B-flat major, D. 471 -- this is the one completed movement of an unfinished string trio.

Glazunov, "Interludium in modo antico" from Five Novelettes for String Quartet

Schumann, Piano Quartet, third movement. This one includes piano, but each of the three string instruments has at least one extended solo where the other instruments mostly stay out of the way.

Borodin, String Quartet No. 2, first movement and third movement. I'm not sure if the first movement is too busy; the individual instruments do come through quite easily. I'm trying to get a sense of how busy is too busy for your purpose.

If you're looking to explore a bit more, there are a number of YouTube channels for chamber music concert series or festivals, where you might find what you're looking for. Some of the videos I've posted are from those channels.

One more thought: search for the Danish String Quartet. They've arranged and published a whole bunch of mostly Scandinavian fiddle tunes for string quartet, and frequently play them as encores. There are plenty of videos of their arrangements on YouTube, both played by the DSQ themselves and by other quartets. Here's one: