I did just notice I have to make the triangles more visible on action popups, yeah. Not sure about swapping to semicircles, I feel like the tutorial saying what they are will suffice. They used to be semicircles, I changed them to triangles because it was hard to space out the semicircles in a way that's readable at a glance + looks good.
The orange lines are pretty much the most important piece of UI in the game. I'm still working on exactly how to present them, but no, they can't be subtle. And a sorta soft goal for the game is to have a limited palette, so I'd like to avoid transparency.
The action economy is also a really important part of the design. I want the player to think about actions as ways to manipulate the situation, usually not ways to directly make progress. There's a reason your attacks all deal 1 damage and your reactions deal 3-5.
There's three phases to the boss.