I actually agree. No way in hell that happens without major changes like you mentioned.
It's good to see that you are aware of the problem. But allow me to complicate things further.
Weapons are tools of change.
Here is how I though to solve it. Let's have two major empires controlling most of the world and they just decide to throw their arsenal of weapons away and use older stuff. Problem solved.
Except that Newton's laws still exist and we already made smokeless self contained rounds, basically battle rifles, and so any state, no matter how small, can start making enough weapons to start upsetting the balance and then it would continue to grow to the point that either the two empires have to readopt the new weapons or get swallowed.
This is just how the world works. Similar to nuclear power and how once someone got it the bigger states had to get it to preserve balance and then ban others from using it.
OK. What about religion?
Well. Again no religion can actually hold people from doing what is practical because warfare is already messy and already involves a lot of nasty stuff.
So no soldier will care about your god if the enemy is using battle rifles and they are charging them with horses carrying spears.
And this again runs to the fact that unless you insure 100% human compliance then just one single state can upset the balance and they we are a race to develop air craft carriers and reinvent history.
I think you have to decide on other limitations other than human tendency.
In fact Sun Zu for examples is adamant about how much of a burden on the state war is and how it is far far better to conclude it as fast as possible rather than belong it and overtax the state. This is actually a blessing in disguise and it is true that modern weapons are insanely deadly yet actual wars now are a lot scarce just because of how deadly the whole affair is.
Anyway I think if you want more clever limitations then you have to go to materials, economy, magic, scientific instability when it comes to weapons.
scientific instability when it comes to weapons
This causes all sorts of problems because the reactions happening in a modern automatic rifles is basically chemistry and physics. So to have them be unreliable then you would also have unreliable life in general.
So tread with caution. However if you are willing then perhaps elements are not as consistent and we can't just create a bullet that works 50% of the time let alone 100% of the time. Maybe a global event, magical or natural, accord and then we from that point onwards when you squeeze the trigger there is a chance of a large explosion or the gun not working or anything similar.
If modern weapons are that unreliable then maybe, just maybe depending on other factors, they won't be used.
Another solution is constant solar flares with massive EMP constantly hitting the earth. Or even modified solar activity to just burn out all the advanced stuff.
Bye bye complicated electronics. However rifles would still work.
As the famous quote goes: Sir Isaac Newton is "the deadliest son-of-a-bitch in space" Expect also on earth.
I mean consider an aircraft like "Lockheed Martin F-22 Raptor" with a cost of US$150 million per unit.
Now before I talk about how unreliable it would be if you are hitting it with intense EMPs, or whatever, you have to understand that it is made by even more complicated stuff that not make it if it is breaking down all the time.
And even if you have a unit then fly it with a 20% chance of falling and dying like very time. It won't be practical.
In that scenario the focus of all human scientists would be to solve the solar flaring issue not just because of warfare but because of all awesome modern stuff is.
Materials is easy. But I just thought of a twist on it. What if the materials for weapons in your world is something organic or something very rare and useful for human survival?
So this is pushing it I know but here is an idea.
All weapons use element X which is actually required by humans to survive this world.
This could be natural or recent event, war or magic or even galactic.
And with the element being rare in the first place it insures that you only have enough to just protect the people.
Perhaps something to do with blood or food or whatever suits you really.
Again this could be planet wide reason for people to constantly use element X, in another form how you would use it in weapons making, so when people started using the element for weapons it started causing troubles world wide.
I thought first of food but a rare element is better here honestly.
This could be even done under a global treaty. The last great war caused such a shortage of element X that the disfigured children of that era still bear the scars.
Military either continue using it and jeopardize their own states and people or bugger off and get back to good old smacking with clubs.