I'd say it's worth playing both against a human opponent and against the AI. The human opponent is more reactive and challenging. But you can get in more turns a day against the AI. I think the only way to learn the game is precisely the same way you learn anything else. You put lots of time into it.
Of course one additional thing you can do is to supplement your gameplay by studying tactics. I made a post that I thought offered some decent tactical advice a while back:
If you're really crazy you can even go directly to the doctrine manuals. For WW2 doctrine I usually go to the Nafziger collection:
https://nafzigercollection.com/product/american-tank-company-tactics-fm-17-32/
https://nafzigercollection.com/product/us-armored-infantry-battalion-fm-17-42/
https://nafzigercollection.com/product/british-and-commonwealth-armored-tactics-in-wwii/
https://nafzigercollection.com/product/british-and-commonwealth-motorized-infantry-tactics-in-wwii/
https://nafzigercollection.com/product/employment-of-tanks-with-infantry-fm-17-36/
https://nafzigercollection.com/product/german-panzer-tactics-in-world-war-ii-combat-tactics-of-german-armored-units-from-section-to-regiment/
https://nafzigercollection.com/product/soviet-armored-tactics-in-world-war-ii-the-tactics-of-the-armored-units-of-the-red-army-from-individual-vehicles-to-battalions-according-to-the-combat-regulations-of-february-1944/
https://nafzigercollection.com/product/soviet-infantry-tactics-in-world-war-ii/
I'd recommend FM 100-2-1 for Cold War/modern Soviet/Russian doctrine. And FM 71-1 or FM 71-2 for Cold War/modern US doctrine.
