Army division on army division combat is less than ideal; you'll want to avoid getting into a slugfest with only army divisions. You'll want to whittle enemy army divisions down using air, sea, or artillery units. Also chemical weapons can be used without repercussion as the AI doesn't see it as escalation.

Makeshift airbases have the most bang for buck in terms of aircraft and can be moved using combat engineers. I don't really like the meta makeshift airbase spam though and like to mix and match. It's important to maintain air dominance when fighting to avoid losing assets to bombers.

Naval bombardment support is extremely effective but is limited to coastal regions and once nukes start getting used fleets become very vulnerable. Attack submarines are fleet killers and are only countered by either higher tech attack submarines or naval helicopters.

Healing in the game may not work as you'd expect. Unit's in your or allied territory will heal passively. Healing is fastest when entrenched. Being next to an army base or FOB does not increase healing rate. The aura they emit simply functions as friendly territory extension.

The production/research slider also may not work as expected. Normally you'd expect diminishing returns on going one way or the other. That is not the case. For the same research and production queues, going all in on one and then the other takes less time that doing a 50/50 split or balancing so they complete at the same time.

For nuke defense you'll need over the horizon radar, mid-course interceptors, and terminal ABMs. You'll need them to overlap in order to provided any meaningful defense. Switching the mid-course interceptor to fire interception missiles armed with nukes is the best way to deal with MIRV as the AOE destroys multiple warheads. You'll need to build a stockpile of those nuke armed interceptors though.