The issue with the last iteration of this suggestion wasn’t that it lacked a melee attack; a boss can have only ranged abilities. The issue was that the idea was underdeveloped.
In this iteration, I think you have the opposite problem. You’ve listed a ton of relatively generic attacks. Most of them are redundant, simply doing damage via a projectile or melee hit. For a boss like this, it’s not enough to just damage the player with pretty effects and cool animations, sometimes inflicting a status effect. Mobs, especially bosses, need a hook - a core gameplay and design element that makes the mob interesting. This can be a weakness or a strength or a special role in gameplay. All of the mob’s abilities and attacks need to play off of this hook and support it in interesting ways. Players should be able to learn and understand that hook and develop new strategies and ways of playing around it. A mob is not just new because it looks different and has a different name, it’s new because it makes players interact with the game in new ways.
Take the grottol for example. It’s hook is it’s a mob that you have to chase down to get a reward - something Minecraft didn’t really have before. It’s resistant to ranged attacks, forcing players to actually chase after it to hit it with a pick. It can hop into Minecarts to get away. It can burrow and disappear if chased too long. Its design is cute and its death animation is sad, making players feel a twang of guilt after chasing it down. All of these things support its hook. (It has a secondary hook too revolving around its living ore theme, which allows you to use silk touch and fortune enchantments when you kill it.) And that’s it! It has no combat abilities, because that wouldn’t support the hook.
Now take Barako. His hook is that he’s a powerful ranged opponent, but he can’t move. So he mainly deals damage from far away with sunstrikes and summoned Barakoa minions. Players will no doubt close in to hit him, so he pushes them away with a solar flare which has high knockback but barely any damage to put them back in his sunstrike gauntlet. The sunstrikes seem to be a basic ranged attack at first, but if you think about it, they come from the sky, allowing clever players to avoid the onslaught by hiding under or building a roof. This is a valid strategy, but once Barako loses some health, he gains new abilities that expand his hook and throw a wrench in players’ plans. His solar beam is line-of-sight based but slow to focus on a target, getting those players camped out in front of him under a roof with a bow but not those running quickly through his sunstrikes. His supernova move was developed specifically to counter players who wall themselves in next to him under a roof or below his throne. It deals massive damage around him, but only after a long charge up time that allows players that haven’t blocked themselves in to get away. It only lifts mobs up and towards him to damage players that dig far down beneath his throne and try shooting his butt with arrows. Every ability has a purpose that works together and supports the hook. It’s streamlined: only 5 abilities. If I gave Barako a big wooden club to wield, it might look cool but it wouldn’t support his hook at all - so I didn’t.
Designing mobs is hard and takes time. But I don’t need a full design from a suggestion. Some of the best suggestions I’ve received were just cool hooks I hadn’t thought of. Think really hard and find that hook.
I've never thought about that before. Honestly, my creation process is, "Hey, that's a cool idea, I'm gonna run with it!" Which is obviously not the best process. I'll try to take your advice to heart and improve upon the Emissary (because I'm quite fond of it's concept). All I can hope for is that it turns out good!