The official rules for threefold repetition indicate that the same position can't happen three times in a game. (https://en.wikipedia.org/wiki/Threefold_repetition) I don't have enough memory on the NES to keep track of all the positions that have been played through the game so I decided to scrap that rule.
I think it could be a good compromise to check that the same 2 moves aren't repeated over and over, I'm just wary of adding unofficial rules to my version of the game.
The 50-move rule is on my todo list and shouldn't be too hard to implement.
I would consider the compromise you described to be a partial implementation of the official threefold-repetition rule, and in this case, I think it would be better than not having it at all. I suspect that back-and-forth successively repeated moves are the most common way of threefold repetition happening in a game. Since threefold repetition allows a player to declare a draw, and fivefold repetition causes an automatic draw, according to the article you linked, it would be great if you implemented both of those rules in the form that you described, with the compromise.
If you're planning to implement the 50-move rule but not the 75-move rule, then I suppose it will be an automatic draw, since your game allows the player to offer a draw at any time already; is this the plan?
Edit: I suppose there isn't a need to implement both the threefold-repetition and the fivefold-repetition rules if the player can offer a draw anytime, but if the other player were forced to accept an offered draw after a threefold repetition, that's one way it could work. And since fivefold repetition causes an automatic draw, they could be differentiated that way.