For my shed, the building inspector suggested doubling up every third
joist. Not only does it strengthen the floor, but it makes the
bounciness non-uniform. Unfortunately, you'd have to install these
when you build as they'd need to go the full length of the truss to
get the most benefit.

You could add some small steel I-beams at 1/4 and 3/4 span.

You could spray in polyurethane (structural) foam. This won't add
load strength, but may dampen vibrations.

But to know for sure, you really need a structural engineer to look at
it and figure out what's going to work.

As a test, you could try just bracing some of the trusses at the 1/4
or 3/4 point with a 2x4 to the floor and see if it makes a difference.
Use one 2x4 to brace across a number of trusses, and a second to go
from that brace to the floor, like a "T".

Note that truss joists are *supposed* to have cross-bracing between
them to keep them vertical. They have much less bendy-strength than
solid wood joists. IMHO the easiest way to add them is to cut a bunch
of plywood strips as wide as your trusses are tall, custom cut off
squares to fit between the trusses, and use pocket screws to attach
them at the four corners to the two trusses.

I don't know if that will help with bounciness, though.

FYI our house uses TJ-25 floor joists (manufactured I-joists) for 18
foot clear spans and we don't have much bounce in our floors.