I really feel like the bosses you face should not be such that you can just walk up to them and hit them repeatedly until they die, there should be some means of the boss countering your style of attack so that a player might be more inclined to take more than just one approach to the given situation. The ending of the omothol boss fight is something that I genuinely enjoyed as it was different from the rest in that I actively needed to change my strategy or chance death

Perhaps you could add additional rewards for re-defeating the bosses with harder mechanics, like for example, you could get knowledge of a unique ritual or set of spells from re-summoning a boss using more resources to differentiate the harder version from the normal version and then defeating them to claim such knowledge. This could also create additional openings for more lore based content, while increasing the challenge for players who desire a good fight and some additional lore

Or bosses which inflict curses on you which require either serious protective rituals to avoid, or additional advanced side quests to remove.

I will say that J'zahar is kind of pathetic.
He has high damage, but a bow Enchanted with Power and Knockback makes this irrelevant since all he can do is whack you with his staff. Poor guy needs a ranged attack. Dreadbeast-man too, but he spawns dreadspawn under you at least if you are far away.

The only thing that complicates the fight is that one of his guardians may spawn, but that doesn't seem to happen is you stay in his little chamber.

J'Zahar is better now; he shoots Wither Skulls and opens black holes!

The idea of killing eldritch beings with normal weapons is a bit weird overall. In H.P. Lovecraft stories, usually eldritch beings are defeated with knowledge- incantations, rituals, and the like.

Perhaps if bosses were made a lot stronger (to the point where they're almost unbeatable), but there was a way to bring them down to normal power levels, you could make things feel way more interesting.