Hi Grenadier,
You can set the difficulty manually, as you already noticed the difference between FM and other levels is the XP gain penalty.
General is EASIER than Field Marshal, but it is also a bit more enjoyable. After 1942 the DLCs become more difficult and replacing units is a nightmare with the 50% xp penalty.
But FM reduces XP gain for a reason, General makes your units much stronger earlier, they take less losses and regain xp faster, it makes the game really a lot easier!
So maybe try FM, even if the penalty is annoying. It's up to you, I am often undecided between General and FM myself.
Regarding Rommel, Guderian, Manstein: I find the later year DLCs challenging enough. Guderian makes some scenarios extremely hard and Manstein somehow plays like an entirely different game as you have to fight very differently. Rommel on the other hand - I didn't try it! - sounds very challenging but in a punishing and annoying way with all the restrictions.
Regarding Strategic Bombers, besides busting ships, their suppression and relative resilience to flak fire make them very good. They destroy ammo and for units with only 4-6 ammo an experienced bomber is a nightmare, as it leaves them stranded with 1-2 shots left, so attack them while suppressed and let them waste their last shot and then go in for the kill e.g.. While you are going to play the Germans mostly, if you are playing against them, strategic bombardment of Tigers is often way more effective than a tank busting dive bomber attack.
Also, Gslaspshot said it already, in Panzer Corps you usually don't have races against the clock and more varied bonus objectives. Rushing ahead and blocking enemy cities from buying new troops is often not as necessary or rewarding as in Panzer General, as many DLCs have scripted unit spawns-