Your problem as you say is you want to keep a good relationship with the founders. While others say there is no value in that because of their past performance, I think if it's not necessary to burn bridges then why do it? Who knows, they could provide a good reference for a great opportunity for you in the future.
So, my advice for you in this case (as an employer) is that:
- Give advance notice that you are thinking of leaving. 2 months would be a good amount if you can. That should give them enough time to find a replacement. It doesn't have to be 2 months though, but the earlier the better. The worst thing you can do is keep all the frustration inside until you can't take it anymore and then have to leave right then. Rather, you should talk to them about it .. just telling them that you are thinking of leaving might be the kick in their backside to get their shit together. (But I'm not saying that is the end goal for you, the end goal is to get out with relationships intact.)
- Just tell them that you want a change, want to try something else, find a new opportunity. They should understand that. They shouldn't expect you to give them X years of your life as if they owned you.
If you do this, then there is no cause for them to be angry at you.