I want to know, if Garrett Wang could act the way he did in Timeless, what he was doing in all the other episodes.
Being a mannequin along with most of the rest of the era?

TNG started the routine of "Data is by far the most human character", a jibe started by people who thought TNG's crew was comprised of robot-like beings. VOY often upped the ante in that regard. (It now makes more sense as to why most modern sci-fi feels "too contemporary"; if 80s, 90s, and even 60s Trek were often showing people "too formal" with verbal speech patterns. Thankfully my gripes at the time had nothing to do with
that...

)
Harry's defeatist/nihilist episode had a lot of robust acting from the get-go, but 2:00 onward - especially the end - and even in this half-contextless clip alone, still does something for me with how Kim reacts to (a somewhat short-lived success). Out of context entirely, one usually says "YES!" like in that shampoo commercial from the 1990s...

but that can't apply there...
IMHO, I have more to ponder and pointlessly worry about as to what and why they'd put ship electronics with all the blinky lights in the actual chair - one snap during an attack and whatever the systems with blinky lights are controlling would be disabled... must be gravity and/or bathroom control...
Okay, all that aside, in seriousness, it does seem to boil down to scripting and that "too formal" stuff, which would make any actor seem wooden. At least with Vulcans, androids, and lifelong assimilated-but-newly-freed-from-the-cult Borg Drones there's a halfway decent reason for using it more sternly, but one prevailing idea I vaguely recall reading (
and may have been from audiences at the time countering the others saying "Data was the only human" shtick) was that 24th century humans were proto-Vulcan. Heck, TNG didn't get any character getting to emote under duress. Even when the Borg came, someone would spout "Course blah blahblahblahblahblahblahblahblahblahblahblah mark blahblah" and it's done so slowly that the Borg could have fried the shields and beam them all into the cube before getting to say "--gage"...
But VOY did miss a beat - if humans were proto-Vulcans or whatever, being lost so far from home and having to stay together and not fall apart... we should have seen more episodes like this one and from more crewmembers and a little more often.