Can anyone help me with these triplets? I want to make them consistent—either all with brackets or none. Thank you!
The only global option available is this one:
“Only when necessary” will display brackets when the tuplet is on the note side, and no brackets when on the stem side. So it’s like in your example, it won’t be the same everywhere. “Always show” will display brackets everywhere (so it will be consistent).
What you can do if you want to remove all the brackets is select all, Filter >Tuplets if needed, and in the Properties Panel set the bracket property as you wish: it will affect all the tuplets regardless of their position or the above mentionned Engraving Option.
Brackets on the beam side would look weird, IMO. It’s entirely traditional for tuplets to be “non-consistent” depending on stem direction and other things.
Or, in some cases, when the tuplet begin with a rest.
But, neither I don’t like very much brackets in general.
Me neither! Probably because in published classical music, even on the note side, brackets are actually quite rare. Many publishers avoid them, like here Henle:
There was a discussion about that two weeks ago 
Thank you so much, everyone! I’m learning new things every day!