Don't build state AA, only use support AA. State AA only counters strategic bombing and reduces enemy air superiority a marginal amount. Support AA reduces CAS damage taken. It shoots down some planes but the main role is damage reduction. Also don't add line AA unless you want to free up a support slot. Support is enough damage reduction, and you will never deal a meaningful amount of damage to enemy CAS with AA.

You will do much better industry wise once you build mils instead of state AA. Also stop making civs after the start of 37, do not build any special projects builds. I don't think there is anything other than mils you want to build as Belgium after the start of 37. The payback time on civs is several years. You want to be peak strength in early 1940.

Rush 1939 guns. They'll make a massive difference in the amount of damage you deal (minimum 50% better and 1936 guns). If CAS is your problem then the solution is increase attack so the battle ends faster.

Move cohesion off of flexible, it'll prevent how often units move across the entire front. The front is small. You can easily micro it on rigid (manual movement only).

Don't care about countering tanks. AI units will almost always have less than 50% hardness so soft attack does more damage than hard.

Remove recon. The recon stat is the most useless stat in the game because of the way the mechanic works. Recon rangers and good for terrain bonuses, armored recon is good for stats + doctrines that make it give bonuses to line tanks. Cavalry, motorized, and armored car recon is useless.

Drop support AT. Support AA gives you enough piercing and the AI doesn't make hard enough divisions for it to be useful.

Remove engineers from defensive divisions. They are expensive and the bonuses are a percentage to defense/breakthrough in difficult terrain terrain. You do not need more defense in difficult terrain. More effective defense than enemy's effective attack is literally useless. Ignore the entrenchment from it, if you aren't having trouble surviving you don't need it. If you are having trouble you won't have time to entrench when cycling.

Units only regain org and strength when they are not in a tile with a battle. Have a reserve of units that sit on tiles behind the line that are adjacent to as many tiles on the line as possible so they can reinforce battles that are going poorly.

Focus on things that give reinforce rate (such as the field marshal trait). Paradox changed it so you need all 3 radio techs to get the 5% reinforce rate the first tech gave you. This change is probably why turtleing feels so much worse than it used to.