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