I seem to be experiencing trigger finger (limited mobility, locking joints), so I’m looking for tools that will allow me to rely less heavily on gripping and holding small pieces.
Is there a tool that will hold the piece in place (e.g., so I can sand it)?
For extra context, some of the pieces are as small as 5mm across.
There are a whole range of tools used for holding very small pieces, many of them used by watchmakers. Which device you use depends on the shape of the piece.
You should pick up a ring clamp or two, a few pin vises with collets of various diameters, a German pattern hand vice, a guillotine clamp (a tool designed in Van Cleef & Arpels shop. Otto Frei sells versions made by Knew Concept), and a few smooth-jaw parallel pliers. You can grind the jaws of the pliers to the shapes you need and line them with leather to keep them from marking the work.
for a very special tool, you should try something that I used. It’s called “a surgeon’s, tool-set“!.
I was back at the ‘setting bench’ after a few days post-surgery!