Launch after selecting "launch ATGM" or "R-21"

First in the numbers is the last one to fire, and the last in in the numbers is first to fire.
AG1 activates ejection motors on R-21,
AG2 jettisons all that - bigger R-21 missiles have their jettison input in the pylon, while smaller ATGMs have that in their internal detacher due to lack of pylon.
VTOL Up is to hoist the ordnance measurer to a greater height

Parts that need a bit of editing when installing these on other aircraft/vehicle:

1:

Main boost/sustain jet, eats about 4 liters of your own fuel per single launch, boost phase is 4 seconds and sustain phase is 8 seconds for R-21, look for 2 numbers in the input. Is tuned to throttleresponse of 20, you better increase that to 100-200 for better ground/underwing launch.

2:

Ejector jet, eats about 0.5 liters of fuel per launch, is edited to work like a quick powder charge, look for 1 number in the input. Its for pushing the missile down from underside of the plane's fuselage, so you better remove this one if you'll do the launch from under the wing or a ground vehicle.

3:

Missile itself, you need to edit 1 number in the Activation Group - but be wary, and edit that AG through Overload, since current default code editor may fail to save the edits made in AG, despite doing just fine in the normal inputs.
Other variant for you is just to preventively save 3-4-more such missiles of the same model but with different sequence numbers, and in that case you don't need to edit anything.

So these are some of my fresh realistic missiles for my finished fighter and some other planes. Normally i'd use an old, detacher-related bug-turned-feature for activating stuff on my ordnance, but I wanted to make some fast, but somewhat realistic "solid-fuel" missiles that work even after their engine is expired, and that start to steer to the target with some delay after engine ignition.
So, i took turbojets for this duty - and since unlike rotators and basic rockets they do not care about detachers, I had to find a way to manually designate their launch sequence, so that the ingame system of doing so wouldn't mess that up.
I accomplish this by adding a secondary missile(R-21 Counter)/bomb(ATGM Counter) to each ordnance unit, so these secondary munitions would act as an ammo counter instead of a actual, primary munition. R-21 has that in the front part of its nosecone, while ATGM has it right behind its nosecone.
So, the actual munitions are only activated in a specific user-designated sequence, one at a time so the game wouldn't dare to launch the other one instead without firing its engine while the one with a working engine just remains attached to a plane. Also these secondary "counting" munitions better be made unlaunchable and undroppable by one and another way. You might see that these missiles utilise alot of inlet fuselages - that ensures that their thrust remains the same at most normal altitudes.

Why would I choose this particular method? Because all-FunkyTrees "missiles" are mostly a no-go for me, since these require even more parts, bear a shipload of coding, and also must be dragged in and out of subassemblies separately from their flight computers which happen to be necessary for their functioning, unlike the aforementioned solution.
Using ingame rockets for propellant is not good as well, especially if you need a big amount of thrust, and worse so if the missile is anything bigger than 50 kg - my 400kg R-21 would need more than 20 such rockets per unit to deliver the desired thrust and acceleration, since the thrust of these rockets is fixed at 500kgf and is unchangeable. Oh, and these rockets also smoke until they (self-)explode.