Is anyone else having issues with the controls? I am constantly finding myself accidentally using paint when I mean to cut, or blue when I want orange. It's extremely frustrating, especially since I never had a single problem with the original Picross 3D.

Has anyone else found themselves getting strikes only 3 moves away from completing a puzzle because they pressed the wrong thing? Has the dual-color mechanic complicated things too much? Or am I suffering old man syndrome as I waste away in the final months of my 20s?

Nah, it's not just you. Practically the only errors I make are due to control issues. I have it set to circle pad-only, since using the L button is cumbersome when trying to one-hand a 3DS, and the cut-off between flagging and painting is quite thin.

I'm okay with the dual colors, but the part that gets me is the visibility on the numbers because of it. It is sometimes hard for me to determine whether I'm looking at a number with a circle around it, a square around it, or if it is already faded against the backdrop of a like color (orange on orange in particular).

Because of these two things, I am finding myself slowing down so that my hand doesn't move faster than my brain - an issue that was not present in the original game. To be fair, that happens with the standard picross games on the 3DS as well, drawing outside lines I did not intend to. There really aught to be a zoom option to help compensate for the bottom screen's image quality and the lack of fineness of the default stylus. Normally I would try to avoid using hint systems, but it is welcome in this case because of how awkward it is to scroll around and decipher the numbers to find that one move you have available.

I dislike the grading system because of the control issues. It makes sense to penalize strikes, but there is nothing more frustrating than making an error 15 minutes into one of the later puzzles and having to restart for the highest medal.

I like the game but probably less so than its predecessor. I'm honestly just trying to rush to finish (I'm on book 40-something) but I have a hard time squinting at these larger end-game puzzles for long periods of time before either my eyes or my wrist gives out (the 3DS XL is not very one-hand friendly for long stretches).

Yeah I've had to redo a bunch of puzzles after like 8 minutes because I pressed the wrong button. Gotta slow down and make sure I'm remembering the shortcuts correctly. Also I can't imagine playing regular picross with a stylus, shit gets so tiny.

Sure does, especially in the later games where they added 20x15 puzzles. Here's an example from Pokemon Picross (not my image, googled it)

I've definitely had this issue. I haven't mixed up painting vs. flagging, but occasionally my brain does a 180 and I'm thinking "blue" but I press the button for "orange". I think I once or twice also removed blocks instead of painting them orange. Might help if there was more feedback (background color? non-intrusive sound effect?) when a "tool" is active because I'm never looking at the icons.

I'm still waiting for my copy of the original Picross 3D to arrive; what were the controls in that one?

I mainly wish you could just turn off strikes in this game (i.e. it should just let you get stuck if you screw up, but you can also just undo your last tap). Though I suppose it wouldn't quite work with the way things "lock in" upon correctly completing a row / column.

No strikes would also solve the occasional issue where I mis-tap when removing a block and it works...it's basically a free block that I didn't want, and sometimes it could trivialize an interesting puzzle (I usually restart when this happens, though sometimes I can see that I could have deduced that block easily enough).

I have not had a problem with the tap targets being too small (it was definitely an issue in Pokemon Picross!); mistaps are typically if I moved too fast or miscounted something. The slicers seem like a decent way to target the right blocks. It would be nice if there was a way to quickly "change axis" around a given block (no idea what the controls would be), and occasionally have found myself wanting lateral slices.

Despite all this, I'm loving this game. I'm about 20 books in and happy to see there are still a bunch to go, and that the puzzles take a decent amount of time to solve without getting overly frustrating.

I'm at the end of book 15 and have found myself making some dumb simple mistakes. I'll occasionally break a block when I meant to color it orange (I only confuse this with orange, never the blue). And as @ktargo mentioned, I'm getting tangled between a painted blocked and a marked blocked (square outline) sometimes. And sometimes I find myself confusing whether an entire row/column is solved or not by how faded the numbers are and I accidentally destroy blocks thinking the entire row/column was solved. I've been doing them all on hard and have been trying to get good times so I'm constantly rushing through it but by now I find that even if I get one strike I'll still get the highest jewel so if I'm already like 10minutes into one and I make one of these dumb mistakes then I just let it go.

If I remember correctly, your only tools in Picross 3D were paint and break mapped to two different directions on the d-pad. The original game did not have multiple colors in the puzzles, you only had one number at the end of any given row or column. It still used the circle and square icons though. So that one was actually more like traditional picross in three dimensional cubes. The concept of flagging and multiple colors is unique to this game.