Pretty cool concept! I think that depending on your model availability, there are ways to do this more effectively. Given that your opponent is expecting the knight and prepping for it, I think you need to have a solid list if you want to win this because so much of your army is tied up in one unit.
Personally I think you could make a cool army using militarum tempestus scions with plasma, maybe one with melta, and a command squad w/ plasma and a tempestor prime, and taurox primes. Half of the army could deep strike to unleash those rapid fire plasma guns and get picked up/supported by taurox primes putting out a wild amount of shots. The elite force being tasked alongside the knight seems to fit well with the theme.
But(!) you can absolutely make a good footslogging guard complement to the Knight also, if that's the flavor you are looking for.
I see below you made the detachment Vostroyan. I like the range gains on that and Tallarn for this set up, the mobility the Tallarn give you is nice as transports and objective-takers. Also, you don't meet the requirements for a Brigade (+9CP), but your 2HQ+3Troops gets you a battalion(+3CP), seems to be a typo.
If you go Tallarn you could probably fit in 3 Scout sentinels (for good deepstrike denial) with lascannons(that don't get the movement penalty for moving due to heavy weapons) by dropping the valkyrie- which is honestly not my favorite unit. You pay a lot for a very fast transport that can't maneuver, and that hits on 5+ ! when it's moving- to fill some fast attack slots if you're eager for those
CP. Use a Valkyrie for Bullgryns and a buffer or two like celestine and a priest if you want to scare your opponent with a quick threat in their midst, but for a kamikaze it's a big point sink that's spread too thin to hurt if the vets want to be within 12" and the Company Commander wants to punch things (and give up his ability to hide as a CHARACTER.
- Astra Militarum Brigade Detachment -
HQ: Company Commander - Plasma Pistol, Power Maul, The Armour of Graf Toschenko
As tempting as it is, this guy isn't ever going to be a melee beast, and the armor doesn't save you from the kind of firepower that is aimed at something like that. You'd be better off keeping your commanders relatively safe away from melee, maybe a plasma pistol but otherwise save the pts. As for the Relic, take a look at the Dagger of Tusakh (deepstriking, if you don't want to grab scions) and the Laurels of Command.
HQ: Company Commander - Shotgun, Chainsword
should be fine, although I like the plasma pistol.
Troops: Infantry Sqaud - Vox Caster
Troops: Infantry Sqaud - Vox Caster
Troops: Infantry Squad - Vox Caster
Probably don't need casters on all 3, 2 should last you. Also, I think you're missing out on some solid opportunities without special weapons. They can all be taken pretty reasonably, but meltas are too expensive for these guys if you ask me and the plasma is very very good for its price. Flamers and grenade launchers were brought into a winning tournament list recently, so you really can't go wrong.
Elites: Command Sqaud - Vox Caster, 3x Plasma Gun
Elites: Astropath - Laspistol
Flyer: Valkyrie - Multiple Rocket Pods, Multilaser, Heavy Bolters
Astropath to buff and fire off a smite-lite sounds fine. I do think I might be biased, but the Valkyrie just seems to be very situationally useful for orgryns or a melta squad or something else maybe.
As to what to do with the extra points, I think some should go to improving your troops with special weapons (21pts if plasmas). Beyond that, I think you could afford a barebones Punisher Russ for 40+ anti-infantry shots when moving under 6". Or you could grab more troops, heavy weapons either in your squads or in teams, maybe some scout/armored sentinels (really solid option for tallarn, less for others), or a scion squad with special weapons and a vanquisher taurox prime.
- Questor Imperialis Super Heavy Auxiliary Detachment -
Knight Crusader - Rapid-Fire Battle Cannon
I've never played with/against knights in person, but it seems to me like the rapid fire
BC w/ heavy bolter isn't much better (or even worse) than the avenger gatling cannon on nearly everything. You might only shoot 2 shots with +2S, same
AP, and a 66% chance of doing the same damage or worse than the avenger- best case, it shoots the same amount of shots. Plus its range is double everything else on the gun; with the flamers, heavy bolter, and 12 S8
AP-2 Dd3 stomps to throw around every turn thanks to the ability to walk in and out of combat it seems like you're going to want to be in
CC when you aren't shooting. Those 28 points saved by going with the inferno cannon should be welcome additions to your
AM detachment, or could help you buy more nice things for your Knight.
Whew! All done. If you read this far, I appreciate it. Cool fluff, let us know how it goes!
The executions will continue until morale improves 