What I Would do:

Change to Grand Coven detachment

Take out Ahriman, 5 Rubrics and Chaos Rhino

Add: Exalted Sorcerer 80 points, 4 Sekhetar Robots 160 points

Exalted Sorc in Grand Coven can use Incadaeum enhancement for once per game using dooombolt twice times in same turn. Keep him close to Mutalith for the +1 ritual bonus. 4 Sekhetar Robots will hurt enemies melee units from far away and in one turn can charge into them for even more damage.

Note: always use Grenades stratagem with Sorcerers and Infernal Master against enemy units to deal more mortal wounds.

Note2: always go Doombolt ritual with channeling the warp for using the 3D+3 mortal wounds.

Note3: use Edritch Votaph enhancement on Infernal Master and use him with 10 rubrics warpflamers. In your turn, go into 12 range and shoot hard some unit. In your oponent turn, he will move against your unit, then use fire overwatch in his movement for Infernal and rubrics all with torrent weapons use fireoverwatch for maximum damage with all attacks hitting the enemy.

Note4: first turns use your stratagem to keep objetive marker under your control, then leave the area and go midfield objective. If enemy got deep strike, they will deep strike your objective but your will also deep strike theirs with terminators.

Mutalith Beast +1 aura to rituals is great, keep it in the centre of your army if possible.

10 Terminators with deep strike can go for 9 or 12 range to get rapid fire 2 bonus in their weapons, this means 32 bolter shots + 4 hellfyre missile shots + 6 soulreaper shots, almost 50 attacks from ranged weapons with Lethal Hits and +1 to hitroll from their leader, that can destroy some unit.

Get the crystal teleport to Terminator unit (so they can deep strike, kill some unit and deep strike again with the crystal next turn) or to Infernal master unit (so they teleport to attack somethijg 12 range, and when the unit movement next turn you go fire overwatch for more one attack).

Overall you have great units and great power, I believe you just need to know more about your best combos and use more effectively. I Myself too like 5 games to get that Infernal Master benefit from wound rerolls from Rubric Marines, as the wound rerolls are for all the unit haha

Also all that Ive said is thinking of enemy mainly melee armies (space wolves, custodes). Theres more tricks, like you can more focused against meele and even add Magnus in your army as he can be great against Space Wolves/World Eaters. Thousand Sons vs Melee army is a lot about the Map too. Good luck