As the owner of an incredibly picky senior thoroughbred, I commiserate.
If he’ll happily eat the cubes, my suggestion is to use that as the base and gradually add. And don’t worry too much about sugar.
My guy likes his cubes mixed moth alfalfa pellets. He loves the cubes but they do take him a while to eat so I started adding alfalfa pellets and he loves those too. Plus they soak up well and he can eat more volume.
We don’t currently eat beet pulp, but you can add this as well. I’d add gradually and get him used to the taste and flavor.
Or, jump into feed. It can be anything. What you have on hand (Senior or omolene) and just mix in a cup with the mash. Let him eat that. I find that it mixes in much better with the alfalfa pellets than just the cubes. If he’s eating this, bump up the volume. I soak with hot water and sometimes can’t even find grain.
Another suggestion is fibre beet. This is a beet pulp/alfalfa conditioning feed that may not be the easiest to find in the US (it’s also not cheap), but it has been a miracle for my one gelding (he has chronic hind gut issues and this was the only thing I could get him to eat when he stopped eating last winter—the only soaked feed he will touch). My senior does eat it, but he has a limit on soaked feed so I don’t give it to him currently. It’s very low sugar and my mini who has foundered can safely eat it. Plus it smells like peppermint. I find it really helps with weight.