Currently the distant shores boss is very underwhelming if you have done this quest more than once. It almosts forces you to metagame the ending and when you do, the difficulty is very underwhelming. Especially compared to the difficulty and build up of the quest in the beginning/middle stages.
missing a suggestion here, OP.
Maybe a solution is to do away with the puzzle element and just make it a straight up brawl.
I like that it's more a puzzle because it fits the dynamic of the quest, maybe if it was a bit random of a puzzle like with something else in the middle of the quest, and it wasn't always the same objective, it could be better appreciated.
Aside, I don't think the quest was intended that you would do it enough for it to become underwhelming. Not that it's improvable, mind.
Casual metagaming kills every quest's atmosphere & difficulty. If you come up with none-DM-policing solutions to this, please let us know.
I would argue the large chamber beforehand is the 'boss' of the dungeon, whereas the final encounter is more of an interesting set-piece. If it does start to get stale, you could always try not completing the puzzle and brute forcing it! Having done it once due to a small bug, it is definitely much more exciting/more of a challenge.
Yes, sometimes the puzzle isn't successfully completed, or is impossible to, as a result of poor decision-making...
...and then you have no choice but to battle an ultra powerful boss monster yourselves.