Would it be possible to build a reusable CAPSULE? Obviously, it would be
launched on an ELV and still have a disposable SM, but why not build a
series of capsules that can be used repeatedly and save a little money?

Yes. During the transition period from shuttle to OSP the capsule and/or
service module can be brought back by a later shuttle flight. This would
require the item to be returned, most probably just the service module, to
be placed in a parking orbit accessible by the shuttle after the shuttle has
completed it primary mission.

Once the shuttle is retired a shuttle free reusable capsule is also
possible. If memory serves the Apollo capsules were intended to be
reusable, they were to "thump" down on land and be refurbished. Again, I
ma working from memory, each Apollo capsule could have bee flown up to five
times. Political posturing about whose congressional district the returning
heroes would thump down in led to NASA to switch to a water landing.
Strongly, being heated to several thousand degrees before taking a sal****er
dunk did nothing for the capsules reusability.

One of the primary arguments against a capsule is the need to mount a full
scale search and rescue effort every time one lands. The Navy billed NASA
through the noose for the use of its vessels and people. But, I have also
seen drawing and notes for a controlled landing by an advance Gemini capsule
using a fabric based, hang glider like Rollo wing. Putting the two together
and a reusable capsule is a very doable and much less expensive than many
make it out to be.