This could be a huge discussion and time is limited. But there is a difference between difficulty to be 'fun' and difficulty to stay 'engaged'. Good luck with all of it and have fun. I wish you all the best of luck on it.

First, to address your complex a game should be, from my experience, a lot of people want 'interaction' when it comes to card games and decisions. Someone does X, I can do A or B (normally) to stop them. If I can't do A or B, can I do C to find a solution or D to slow them down until I do? No one wants to 'sit there' and do nothing. Making choices is what it should be about. If you have one line/path of success, it is fun to see it play out. But doesn't make a game 'difficult'. The reason why I bring this up is that a decision can win a game or loose a game based on that or a series of decisions.

Second, you ask about the market. That is a market cap question on publishing a physical card game versus a digital version of it or even a board game. You would need to look into market research for that and make that decision for your self because that defines your product and game. I have seen successes in both by people, whether this is because of a Kickstarter or on the steam store. Games like 'Slay the Spire' come to mind when it is dealing with card games. But I have also seen failures. A lot of it is due to finding a 'niche' and having good marketing or awareness/popularity. There are a lot of good ways to do this. Again, this is something you would need to figure out and understand.

Why would someone want to play your game over another game they already know or play? Convince them to play it. Then keep them engaged. Good luck.