Should I build my army around the triangle's ±7% bonus?

No — that's a beginner's mistake. The troop triangle's ±7% is real but small, and above all not controllable against an unknown target: the defenders' type is random, so you may be at +7%, -7% or neutral.

When the ±7% *is* actionable: only when you know the enemy type → in practice, small tiles (below level 10) and PvP you've scouted. All the high-value content (10+ tiles, beasts, wandering knights, points) means an unknown type → ignore it.

The mistake is recomposing a whole army to scrape 7% off a target whose type you don't even know. You sacrifice far bigger levers (research, +20% affinity, the -27.5% aura).

Rule: ±7% if it's free and you've scouted, never decisive. Otherwise, maximize the controllable.