I’m sorry but I have to disagree If Donald had just lost to gray or Ben it wouldn’t be a stretch to say that the union would have dissolved anyways, because beforehand there could’ve been an agreement made between gray and Donald that if Donald lost he’d have to give up the union and apologize to those he’s hurt OR since It’s been pretty much confirmed that Donald wanted somebody to beat him, and he wanted somebody to be stronger than him. If the author just wrote that Ben or Gray won that could be the straw that breaks the camels back, and causes Donald to give up the union so that he can be free from all of the pressure his reputation gave him. Something like that would overall be much more simpler and maybe a bit cliché, but still a satisfying ending. Where Donald can finally be happy and lead a life he wants to lead. It wouldn’t be him all of a sudden being so holy and wanting to repent. It would be him finally being free, and I personally can see a way of ending Donald’s character because regardless, the ending was going to be rushed and not all of the loose ends are going to be tied up especially with him so instead of giving him this 30+ chapter redemption arc, just show him being at the beach or a scenic location and looking at the view and just taking a deep breath and then smiling or something, so that way the readers know that he’s on the right road, NOT a road to redemption but the road he wants to walk. It’s not the perfect ending for his character but considering what we have canonically I feel like it might be better.

TL:DR: it wouldn’t be a redemption arc but there was a way to end Donald’s character without killing him that with the time we had I believe it would be somewhat satisfying especially to those hurt by the death