But actually a lot of what we’d need is much easier to mass produce and research than you think it is. Like your average artillery, armor, and infantry unit basics.

That's true, so we'd need more details to discuss specific spending and costs.

Also, it doesn’t need to be a two front war. We have an entire ocean protecting us on both sides.

The naval and island hopping campaign battles for the Americans in WWII seemed like they had to happen. I would prefer that the battles take part in the open ocean than in the homeland, though I wouldn't want the battles to take part in islands of allied and neutral countries where the locals have to pay the toll either. Still, it seems like a war with China and Russia at the very least would take part in Europe and the Pacific. Perhaps Africa will be a theater since Russia and China have been developing a lot there. In fact, Wagner Group (the Russian mercenaries that was lead by Prigozhin) has had a presence there for years now. Regardless, that's only China and Russia.

If North Korea joined, then we would include the Korean peninsula, of which North Korea has spend decades preparing for an invasion by digging tunnels and setting up other defenses while their population is brainwashed to fear anything that is not North Korean. If Iran jumps in, then the Middle East including the Persian Gulf which would be an important theater because of energy/oil resources. Basically, a war like that would have the capacity to involve more than two fronts.