Please offer some advice.

I think the comparison to chess is very good. However, it rarely works out to develop your strategy exactly as you planned. A lot depends on the ALTARS (for me) and the magic available in the castle. We're playing a map (I'm playing as Nature), and in the castle, there isn't a single buffing spell (like Strength of the Giants). Developing Nature magic is going poorly. I have to focus on Strength.

You need to think about it not before the game, but during it, but preferably at the beginning. When you already have some cards open. For example, you already understand something about the castle. Or about the opponent. You say that there isn't a single buffing spell (meaning, no blessings). But what is there? Let's say there are various summoning spells (wolves, fairies, satyrs, etc.). Then you can focus on that in the future. And develop Death or Order magic. Then you will enhance your summoning ability (replenish it with imps, ice elementals, poisonous creatures, and devils, or with illusions). This also strengthens the hero's army; you shouldn't neglect this ability.
In general, you will be able to increase your army threefold. In the Portal, you build Manticores, using summoning to bring in water elementals (especially good against Chaos), and on the battlefield, you summon, for example, Devils or Magic Dragons, with their ability to help the hero with magic and deflect various tricks. That will be significant help.