[2023] is there a reason to finish tier 5 quest ?

I think it just depends on whether your goal is efficiency or the experience. As an experience those quests are pretty epic for a single player and a lot of cooperative fun for a group. But for efficient play and striving to progress as quickly as possible the rewards fall short. So the question is what your goals are in playing.

If you’re going to take the time to go over there, loot everything, and by necessity deal with any zombies that get in your way, it seems like you might as well do the quest and get the reward too. Even without the reward, I find it satisfying to know the building is completely cleared. If you’re looking for a tier 5 challenge in a smaller, quicker package, try an infested quest.