Yeah, you got the point. While maintaining the classic gameplay loop, boards will always be solvable without needing any item :)
Even though I want to keep the basic loop close to classic Minesweeper, you are right that curses are probably the key to keeping the game fresh and really taking advantage of all the items. For endless mode, for example, I was thinking of making curses a fixed rule that you carry from the moment they appear throughout the entire run, with maybe an option to choose between curses and more items that interact with them (remove a curse, reroll a curse, ecc).
Honestly, I think this approach could work in normal runs too, making endless mode just a continuation after completing the fixed floors (as I think most players expect it to be).
And I will definitely keep your ideas in mind. As you said, some of them I had already thought of, but having many more ideas to iterate on and test is great!
Thank you for taking the time to give such constructive feedback, I really appreciate it!