Eh I don't know calling something "bad game design" always sounds a bit entitled, many games still have unlockables and it can be fun and motivating to unlock stuff. It's not like what you experience in the second half of the DLC is overall important to the story. It's just forshadowing, a small glimpse to the future of the series. I could say the same about games who force you to get 100% to unlock a special ending like Batman Arkham City, it would be a pain in the ass if the game had forced me to search for all lucky emblems since I never liked searching them.
Also you can still play the game with fast pass and oneshot every boss, so if you really wanna see the secret ending so urgently you can spend some time and do this instead.
The fights were great, but what definitely kept me going was the surprise at the end, if I could just have it from the start it wouldn't feel so tense and motivating.
Btw Kingdom Hearts 3 unfortunelately did not offer a fine balance between both for the longest time. I waited an extra year, just for the DLC to come out. I quit the game intbetween since it was so damn easy even on proud. It infuriated me. Critical was more punishing but the bosses were still pushover moveset wise the only exceptions are the fight against Dark Inferno and Ansem, Xemnas and Young Xehanort. So players who favor challenge were left alone for a long time.