Great, thanks. Say 6 tracks are at the -26db. Is there any difference between +8db each of those tracks using their channel's trim/gain knob, or can I just +8db the master fader's trim/gain?
No.
Are those both equivalent moves?
Yes. The math is weird because of all logs and exponentials involved, but with nothing on the Master that might be level-dependent, it will be exactly the same thing.
It seems like the first one might be a better approach since the plugins would get the volume they need, but I did automation on each track and it might mess with everything.
As mentioned above, it only matters for non-linear plugs - things that compress or distort/saturate/clip/warm. But those plugs almost always have input gain and/or threshold controls, and I'd imagine you've dialed those all in by ear, no?
If I was starting a new mix with tracks that low, I would probably prefer to make this up at the item level so all the track level stuff "looks right". Literally just normalize the damn things and go. In Reaper, this is a non-destructive process - it literally is just automatic adjustment of the Item Volume. You can see and change what it's done to a given item by viewing the item properties. You can normalize from there, too, but it's probably a bit tedious with multiple items.
There's always the Item Volume envelope. I'm pretty sure it trims (adds to) whatever value is set in properties. Both of those actually affect the size of the waveform on the screen.
Then you've got the PreFX track volume envelope.
A lot of people complain that there isn't a specific Pre-FX trim/gain knob on tracks in Reaper, but we've got plenty places to adjust that, and each one is more or less appropriate for different situations. We do also have the JS Volume Adjustment plugin which you can stick anywhere in the chain that you might want, and there's a way to put that knob on the mixer if you really want. You have to be careful with that, though, because it has a built in (if adjustable) hard clipper.
But honestly that's all for future reference in other situations. Your issue now is that you've got a mix you're perfectly happy with, and it's just a little quiet at the Master. You can either dick around changing gains and comp thresholds and automation and basically (try to) rebuild your whole mix, or just turn up the Master fader. I mean...it's your decision to make. I know what I'd choose.