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Yeah, this was hard mode.

But I imagine this is a tough game design issue to solve. Basic minesweeper has a 'one-mine-and-you're-dead' design, and even with that and the occasional 50/50 a good player can solve most fields most of the time. 

Relics, artifacts and health all make it easier to get through the game, so they are an excellent way to scaffold the learning and get newer players going. I really think you are doing a great job with making this system powerful and interesting.

However, making the basic game harder is way more difficult, especially without breaking the fundamental joy of the puzzle, or the ability to solve it at all.

I think you're on the right track with the curses, but only a few tap into the basic setup of the game (limited vision or more damage might be a hindrance, but the game is the same). I think the 'let's remove some tiles' works best, in that it forces a veteran player to reassess otherwise familiar setups.
Maybe you can think of more curses (or just variations of the base game) in that line? 
Let me brainstorm some ideas (not yet filtered for quality or practicality): 

• an irregularly shaped outer edge (diamond, heart, etc)
• tiles that show the amount of mines in radius 2 instead of radius 1
• tiles that show the amount of mines in straight directions and not diagonal (so 4 out of 8 possible options)
• simple '?' tiles, that are in the game but don't give information
• moving bombs
• uncertain tiles, that show the amount of bombs +/- 1
• tiles that indicate a pattern. Like 'I' is that there are two bombs on opposite sides of the tile and 'L' means 3 bombs around 1 corner.
• tiles that create bombs if the surroundings are left alone for too long
• tiles that automatically flag bombs next to it when certain conditions are fulfilled
• a bonus if there is a bomb-moving or bomb-generating system and you get a revealed tile with 8 bombs next to it (another way to make things harder is by adding additional goals/challenges, also like you're doing with the chests)

You probably have thought of many of these already, but I just liked doing a quick game dev exercise for myself ;)

Have fun with development!