Lets agree to disagree on Gamelin just for a moment, maybe fighting from behind the line will by itself force a correction on some of his more pointless adventurism by restricting the front he can play with. He is still an adherent of the Continuous Front doctrine though, he is not going to be thinking of defense in depth with all that implies for an eventual penetration and at it's very widest points the line is 12 miles deep. And we can probably say that the single biggest mistake of all pre-war (aside from building the line in the first place), Allowing Belgium to leave the Alliance and then not constructing a full scale extension to the northern line, was not entirely his doing though he like the rest of the French high command felt that a war of maneuver in Belgium was exactly what the French army wanted.
The question with Gustav and Dora, is could they have withstood multiple hits sustained over a week, a month or more? Concrete is apt to fracture under such circumstances and we enter a 1453 scenario with cannons popping big holes in the walls.
In any scenario where France retreats behind the line and cannot strike at German Industry you give the Germans the luxury of time to figure it out, and having the Czech fortifications to study, which were built with French advisers overlooking them, they can work out roughly what is needed and theorize about how it is all interlinked. They are still going to have Paratroopers and no doubt the idea of dropping in unexpectedly to support an assault.
And as per our timeline you will lose parts of the line and see crossings of the Rhine and unless the interval troops are super vigilant and fight on despite lacking air cover, or you'll see major penetrations and captures of cities (see Strasbourg and Colmar in June 1940) when weakpoints are found even without the big guns being used, and yes they will be found.
Now onto artillery, and production in general. The problems were beginning to solve themselves, between '39-40 the old vices of ordering experimental aircraft and tanks then cancelling production as soon as the next government swept into power and passing production of new units to a favoured industrial concern could no longer be continued with.
And eventually industry understood it was at war and factories stayed open longer and produced more arms. You still have the potential for communist sabotage and strikes, but personally I think these problems were likely overcooked historically.
If you keep France on her feet to '41-42 things will definitely see an improvement in numbers and quality of armaments produced.
Artillery we are fine for, not only new and well designed pieces but a giant quantity of the famous 75's are still available. Anti-Tank guns, and Anti-Air a lot less so.
French tanks vs German tanks, well it wasn't a matter of quality, more a matter of doctrine, tanks were separated out into small penny packets and used as 'plugs' or mobile pill boxes. This frittering away of the armoured forces nullified the advantages they did have in protection and weaponry and left them isolated and unsupported, easy enough prey for the Germans, particularly once they worked out what a terrific anti-tank gun the 88mm was.
On trucks vs horses (and panje wagons for that matter) this actually matters less that it should do in a semi-static war, trucks without air superiority can become a major liability as they can be strafed as they trundle along roads to switch from one zone of attack to another. Here is where the underground railways between the northern fortresses would show their worth allowing the French soldiers to swarm like bees from a hive to any zone under concerted attack. But once any part of the line is penetrated then it is a race against time to counterattack and force the Germans back, because once their Armour is across they will use it better, and will use it with full air support and they will penetrate deeply.
One would think that the longer we wait for the Germans to get through the better prepared the French army should be, with well prepared defenses, yet historically what we see under Gamelin's aegis does not really bear this out.
And our friend Gamelin (I am sorry clearly I have not managed to put him to one side

)in '36 wrote to Sir Cyril Deveril (British C.I.G.S.) stating that the German tanks used in Spain were 'inadequately protected, fit only for the scrap heap' and 'all our information shows it is our doctrine which is correct' ie spreading the tanks amongst the infantry, he had not changed his mind in 1939 or 1940, nor had most of his generals. It's unlikely he'd resist the temptation to penny packet the tanks and use them as mobile petits ouverages, instead of holding them back to use as a mass de mobilisation.
All of this though forgets really why the line exists in the way it does. It exists to deliberately funnel the German army into Belgium in order to force them to violate Belgian neutrality and bring both Belgium and the British Empire into the war, which is a fairly smart strategy if Britain hadn't transformed its army back into a colonial police force and the Belgians are willing to play ball, but it also absorbed the sort of money that would have fully mechanized the French army.
I prefer a different what-if, one where Petain and Gamelin are somehow sidelined early, and Weyand and de Gaulle produce something like de Gaulle imagined in his
Vers l' Armee de Metier.
Imagine Hitler not facing France hiding behind a wall but a fully professional mobile force with the best tanks for the job (lots of training for the new force would expose weaknesses and allow rectifications in design to be made that didn't really happen in our timeline)immediately ready to be deployed in '36, '38 or '39/40, and France is transformed from requiring hope and luck to survive into being a dangerous foe that can sweep into the German rear and force them to fight on two fronts before they are ready or finished re-arming. You have an Army that lets France have a much better chance of imposing her will on the Czech issue, and I'd say with that sort of change France is bringing a gun to a gun fight, not a wooden spoon like Gamelin did.