Looking for feedback on spicing up a Behir encounter

Hey all,

So my party will be coming up to their final encounter in a rather long dungeon crawl they have been in for the last 3 or 4 sessions. They are four 8th level PC's, and that I have been fairly generous with concerning magic items.

I really want to give them something memorable, and my goal here is two fold.

1. I want to really make this a tough and memorable encounter, I have noticed the PC's a "batting way above their average" so to speak, when it comes to Challenge rating, so I really want to beef this one up.

2. I want these abilities to add a sense of depth to the encounter. I want most (but not all) of these additional abilities to be things that the players can notice, and alter their play strategically to counteract/avoid.

That said, here are my changes:

Serathis has the Statistics of a Behir with the following Changes:

He has 203 hit points.

Serathis gains one legendary action, which he may use at the end of another creature's turn. The nature of this legendary action changes based on the chart below, and Serathis regains the use of this ability at the beginning of his turn.

Before guarding the mountain, Serathis made sport of hunting and killing dragons, and has taken on some of the abilities of his foes. Every round, his scales take on the color of a different chromatic dragon, going in descending order from the list below, starting at Red, and ending with White before beginning the cycle anew.

Red: Serathis gains Immunity to fire damage, and he lashes out with a fiery explosion. Creatures within 5 feet of him must make a DC 15 Dexterity saving throw or taking 15 (3d6+5) fire if they fail, and half as much if they succeed.

Black: Serathis gains immunity to acid damage, as he lobs a glob of acidic slime from his mouth at a single target. The glob grows in size the longer it is in the air but starts small, and has disadvantage against targets that are 15 feet or closer to him. +8 to hit, 4d6 + 5 (19) acid damage

Green: Serathis gains immunity to poison damage, and releases poisonous spores from his scales. All Creatures must succeed a DC15 Con saving throw or be poisoned until the end of their next turn.

Blue: Serathis gains immunity to lightning damage, and lets out a bolt of chain lightning. +8 to hit ranged spell attack. 14 (3d6 + 4) lightning damage, and will chain to 2 additional targets if they are within 10 feet of the original. Roll to hit for the second and third target.

White: Serathis gains immunity to cold damage, and creates a storm of ice and sleet that is 40 feet wide and 15 feet long. Any creature that starts its turn in, or attempts to move through the storm must make make a dexterity saving throw, taking 3d8 + 4 (17) Cold damage on a failure, or half damage on a success. The storm is thick enough that it provides Serathis total cover from anyone behind it.

Enrage: When Serathis is reduced to half health, he goes into a rage, and immediately regains the use of his breath weapon. He also gains a +1 to all attack and damage rolls, and his spell save DC increases by 1.

I definitely want everything to be tough but fair, and if there are any huge power disparities, that would be something I would want to fix!

Thank you for any and all feedback!