This is amazing, I love the interpretation of the John theme, and this game is super replayable. It would be cool if the mechanics deviated a bit more from MTG or Hearthstone but the cards themselves are pretty fun. Despite how very clear you were in the game page rules it took me a hot minute to realize you could only attack the space opposite to the John lol. It also wasn't immediately clear that drawing took an action, but that just took me by surprise the first couple times. Other than that, just a small tweak I'd suggest is auto ending turns when the action points are exhausted, and slowing down the opponent's actions a hair.
As far as supporting other resolutions, consider using the RenderTarget2D, you basically set that to your ideal resolution, draw on it, and then draw the render target to the actual screen buffer, scaled to the backbuffer dimensions. The next thing you'll run into is maintaining the aspect ratio, but that'll get you halfway there. If you want some more explanation on this let me know!
Great job on this overall!