The CtC community sometimes overlaps with, and mentions in their vids, https://www.dailykillersudoku.com/ which also has a nice UX, especially for helping compute totals and possible sum-partitions. Some features might bear imitating/flattering.

I like it showing you the possibilities when you click on a cell in a cage, but I was really wishing they had a "press shift and the number to set the cell value" feature. I messed myself up a couple of times by unselecting the wrong number when attempting to set the cell and not noticing for a couple of moves. For example, if a 10 cage is a 28 pair and I figure out that the left one is a 2 and the right is an 8, I was clicking on the left and hitting 2, making an error.

If you hold shift while hitting the number it should just set the cell to that number, not just erase a mark.

I didn't know about that one, looks neat. Thanks.

Sums of cages and selected numbers (even outside of killer/sandwich) were a must have feature for me, so I implemented those features early. Also combinations are available for single cage or multiple selected cages and for selected sandwich.

Something I wish DKS did better was Fill. If you have selected a cage and press F, it will fill the cage with all the possible combinations on the right panel. Instead, I think it should take only the ones you haven't ruled out by de-selecting them.

Thanks, I was matching colors of another popular Sudoku app. Ultimately it would be best to allow users to configure colors and other presentation attributes (thickness of the line, alpha, dotted/dashed, etc...).

+1 on matching the CTC 'standards'. You'll get significantly more user adoption if the conventions match.