I'm happy that bosses don't despawn as I would otherwise need to gather the materials needed to summon them again and start the battle anew with them having full HP, in the unfortunate case of dying.

The aggro is another thing all together, it is questionable if you had your secondary base too near it, as they should be able to wonder around but have some sort of range. Like when events happen, you have the red circle thingy on your map, the same could be implemented with bosses but I have yet to see them wander too far from their spawn, sure, they will chase you.

I usually am over-prepped for fighting bosses, when it comes to using teleports. I setup first one where I make land, another about halfway there and third near the summoning altar. these 3 teleports are the minimum. Prepping to fight moder, I build 9 of them around the altar and peppered here and there in the vicinity.

But now you know something new about this game, and it's all about the journey

Did you manage to take him down?
Do you need some spoiler-like tips on how to beat him easily?