A few things to consider;
Competitively your Tzaangors are better as a single 30 man blob that you either deepstrike or dark matter crystal into your opponent's face turn 1 (with the crystal, turn 2 otherwise). It's an incredibly potent unit as it'll obliterate just about anything in front of it, especially with proper support (strats mostly but Shamans and even Mutaliths help). It also helps mess with your opponent and force him to adjust his deployment to take it into account, which can't be underestimated. Plus, if you're facing a charge heavy army (say, Kraken genestealers or a World Eaters Zerker rush) you can still use them as a screen if need be. Just don't forget they're basically glass cannons; a 5+ invul is nice but is still only going to stop a 3rd of incoming wounds on average. Their strength is in overwhelming your opponent in saves. It's also a reason I'm not a huge fan of them as a screen since they really want to be charging/attacking first.
I prefer Rubrics in 5 man units. I think the SRC is a trap, it's not bad per se just not worth the extra 4 Rubrics it takes to get it. I found a SRC unit will get targeted by my opponent early-ish and it got to the point where they just weren't around long enough (remember, while All is Dust makes Rubrics more survivable it only procs on 1 damage weapons meaning Autocannons and the like will still rip you a new butthole). I generally take 2 5 man units as objective campers and they do good work. They're small enough to fly under the opponent's radar, survivable enough that they'll survive against the small arms that will come their way now and with two units you get another sorcerer for added mini-smite or other power. Also I'd drop the Icon of Flame. I mean, if you've got 5pts lying around it's ok but otherwise I've never found it to be worthwhile as I'd rather upgrade a gun somewhere than have a 1 in 6 chance of a mortal wound. Plus if you're like me you'll probably forget it exists half the time.
I don't like Defilers. I think they suffer from trying to be too many things. They have shooting but BS4 isn't great, even with the re-roll 1s you'd be giving it, and it's got combat but if you're going for that you're better off with another Maulerfiend. I know some people who like them though so it could just be me.
I also wonder if you're got too many characters. For Luke it works because he doesn't take Magnus so he's got that 445pts to bulk out his army. But with Magnus I think you're suffering from a lack of bodies. Personally, I'd drop all 3 of the Exalted Sorcerers. You don't need their psychic abilities realistically, as Magnus+Ahriman is plenty as is even without the DP, and it frees up nearly 400pts which you could put into...well, anything really. Scarab Terminators, more Rubrics, Tzaangor Shamans or Enlightened (they add a lot of speed the Sons lack, should definitely check them out with the bows).
My final point, and one that applies to all Chaos armies, is that it's worth considering bringing a daemon detachment. There's a lot of synergy because of the shared daemon keyword (Magnus and Maulerfiends have them, for example) and it can create some interesting combos. The Changeling is a character, for example, who gives Tzeentch Daemons a 6+ Feel no Pain that can help with Magnus' survivability. Their bogstandard HQs add +1 STR to daemons, so you could make Maulerfiends STR 7 base as well (nothing special there, but does help as the Maulers degrade).
Plus it's fairly cheap to fill out a battalion with them. Take the Changeling (100pts), a Changecaster (65pts on foot HQ, can get a disc for 85pts overall if you want mobility) and 3 10 man squads of Pink Horrors for 70pts/unit (use them as screens instead of Tzaangors, as they can shoot and have a better 4+ invul) and it only comes to 375pts (which fits rather nicely into that Exalted Sorcerers sized hole I talked about above ;p). Too expensive? Go full power gamer and swap the Pinks for Brimstones which are only 30pts/10 man unit (but with only a 6+ invul) and turn it into a 255pt batt. Brims are tiny and make amazing objective campers as they can hide from LOS behind almost anything, similar to Nurglings or Ripper Swarms if you're familiar with them only these are single models and not bases making them even better against anything over damage 1.
And that's not even considering the other units you can bring. Flamers (the unit) have 12inch flamers (the weapon) meaning they're one of only 2 units I can think of that can deepstrike and flame (the other is forgeworld Custodes terminators, which are signifcantly more expensive). Exalted Flamers are similar but also pack an 18inch lascannon alternate mode. Screamers can add a ton of mobility to the Sons, etc. If nothing else it's worth exploring as it adds a ton to the army while still feeling fluffy (unlike, say, a CP farm guard batt in a space marine army).
But honestly, even with all that said, I'd just go with what you're comfortable with and what you think is cool. It's your army and it's your time/money that's going into it after all. And if you're not going to tournaments I think the list you've got would do well enough at a local club.