Also these guys[1] have some apps[2] with some pretty difficult ones (some classic Sudoku and some with novel rules).
Also, if you are a Sudoku fan, you might want to check out kakuro-type games to see if you like those as well.
[0]https://puzzlemadness.co.uk/greaterthansudoku/hard/2022/10/7
[1]https://app.crackingthecryptic.com
[2]https://crackingthecryptic.com/#apps
[3]https://www.kakuroconquest.com/8x8/expert