Now I'm trying to figure out what, if anything, to apply as a polish to give a smooth texture and some sheen.
Note that Minwax Polyshades stain already includes polyurethane in the stain, so I already have that level of protection.
It's not necessary to add anything further when using a 'stain' like that* but it is generally considered advisable on items that will see higher wear.
The usual thing is to add some plain polyurethane on top of the coloured poly so that you have an uncoloured surface layer that can take the brunt of any wear and scrapes. I would recommend applying it as wiping varnish, see previous Answer.
Should I just use fine steel wool now to create a smooth surface and bring out the existing sheen?
You can use steel wool, gently, to smooth the surface prior to applying straight poly if you like, a fine nylon abrasive pad (either Scotch-Brite or an equivalent) or fine abrasive paper are also used.
But I only do it if the surface has any roughness or dust nibs, if neither are present I don't bother because it serves no purpose. It's not necessary to scuff for adhesion reasons unless you're varnishing over an old finish.
Or can I apply something else such as paste wax to help with additional protection and add some more sheen?
Paste wax won't do squat to add protection :-)
Waxes certainly add sheen, but that's essentially all they add. The minor protective effect they do provide is merely to help liquids bead up on the surface, giving at most a few moments' grace time if you have a spill.
The sheen is better provided by a shiny, hard surface coating that you're already using, in this case polyurethane varnish. If you get a good sheen with this (and you've already gotten a fairly respectable sheen from the Polyshades) it will be far more durable than that raised by wax polishing the surface.
*This is a stupid commercial name for this kind of product, the makers should just label it (and market it) as coloured varnish because that's what it is and prevent confusion.
