The principal of ramjets is simple, but you need materials and pumps.
Already developed to a highly sophisticated level... even a motor car has pumps and fuel systems, as do motor bikes and lawn mowers and petrol driven edge trimmers...
In comparison solid rocket fuel is very expensive.
The most brilliantly designed ramjet I can think of today is in the KH-31 where it uses the rocket booster as a combustion chamber for a ramjet.
To be fair the SA-6 surface to air missile has been doing that for more than 50 years... they have a lot of missiles that use the combined rocket ramjet design.
The main alternatives was strap on booster rockets like the SA-4 missiles, or solid rocket stages like the western Sea Dart and a few US Naval missiles.
Obviously strapon boosters make it wider and solid rocket boosters make it longer...
But you will still need air intakes that will complcate tube launch.
Why would it complicate tube launch?
The Yakhont/Onyx/Brahmos has a fairing cover over a nose mounted air intake for its ramjet engine... newer higher speed missiles tend to use rectangular shaped air intakes for better air flow control than older round intakes like the nose intake on a MiG-21.
The rocket today has plenty of energy, and it still has lots of energy when it hits its target.
Having to carry all its fuel and its air supply to burn that fuel makes it heavier and shorter ranged than a jet engine.
Don't you think there is a reason jet engines are used for propulsion for aircraft these days... just having to carry the fuel and scooping up the air on the way makes it much more energy efficient...
The Grad rocket is not a precision weapon and I would say saturation is also a valuable capability although of course a degree of accuracy is required.
What I am trying to point out is that Grads don't need to be turned into single shot precision super long range missiles... the Grads are optimised for use against area targets, where precision is not critical, and a large volume of simple cheap rockets is what makes them effective.
If you want longer range well there is already Uragan and Smerch, and if the target is a single object then Hermes has terminal guidance and will soon get terminal manouvering capability to evade air defences too.
Even 152mm gun artillery will reach 180km with guidance...
The need for new super long range Grad rockets does not really make sense in my opinion... they already have submunition and anti armour rockets and can blanket a wide area in HE and fragments... if you want guided missile long range accuracy then the Kh-50 is a 1,500km range guided cruise missile...
It would also allow the GRAD to fire a much flatter trajectory to avoid Iron Dome type weapons.
A high altitude release submunition warhead would render Iron Dome useless... the 9M218 has 45 HEAT submunitions per rocket out to 30km range, so a volley of 40 rockets means 1,800 submunitions to intercept per vehicle... a 6 vehicle unit means almost 11,000 submunitions to intercept... so how good is Iron Dome again?
Of course the 9N176 is the 300mm Smerch equivalent which has 646 HEAT munitions in each rocket, so about 46 thousand munitions from 6 trucks with 12 rockets each...
It would be very easy to add "non-precison" range guidance with a simple fusing mechanism or airburst the rounds. It would allow them to use the vast inventory of launchers they have today while increasing capability by orders of magnitude.
They have an enormous range of alternatives in their arsenal, and these unguided area target weapons remain very useful in very specific situations.
Making them precision guided will be making the same mistake the west has made... the unguided rocket is a sledge hammer, not a scalpel, and trying to make it a scalpel will make it too expensive to use in the numbers that make it effective.
By all means new types of rockets make sense, but don't lose sight of what they are for.