I have 2 thoroughbreds..one is a moderate keeper(atm FAT!) and the other is a very hard keeper.
The hard keeper is on Tribute Kalm Ultra(12% fat), Finishing Touch(18% fat), along with Alfalfa Cubes and some veggie oil.
Moderate keeper is on Tribute Kalm n Ez(8% fat), a little bit of Finishing Touch, and beet pulp.
My hard keeper does not like beet pulp so he gets the alfalfa cubes for fibre and he has always done better with some alfalfa in his diet. I can't seem to get him to eat as much hay as my other guy either but recently been cleaning up all his hay unlike before. Both get free choice hay and a prebiotic/probiotic.
Id do either a senior feed or low starch high fat complete feed, free choice hay, beet pulp/hay cubes/roughage chunks, and added fat from either oils/rice bran. Keep the sweet feed and slowly introduce the new feed until your no longer feeding the sweet feed.