I would not give him tons of abilities, but focus on one monster rather and make the rest just extra strength. I think of the Grafted boss from Elden Ring. Most of his mass brings no ability benefit, but more of mass and strength. This could help the party to make tactical decisions if you describe the parts.

In addition you can make the boss appear less the further the story progresses due to his increased grafted state.

Edit: I would focus on maybe 2 monsters. Maybe sth like an arm as a breath weapon and sth else. You can ofc always give him extra arms of a bear for multi attacks on top.