With the urgency of the problem, I think we should take as much advantage as possible of any already existing structures, and use them as a starting point for something new and hopefully better

Something like NATO minus USA makes sense. Maybe the demented orange shit stain does us the favour of leaving NATO, which saves the effort of giving the thing a new name and legal framework.

But a purely military alliance based on geostrategic interests alone, like NATO, doesn’t cut it. The EU needs a serious own capability of defending itself, as well, as there is strong political, economical and ideological alignment and co-dependence within the EU. I’d strongly argue for it being a separate arrangement from whatever needs to replace NATO, because geostrategic interests can shift easily. (see USA)

Therefore, the EU as a whole needs to step up its defence cooperation and capabilities, both to deter any land grabs by any megalomaniac imperialist dictator, East or West, and to benefit from the economy of scale. As long as every European country is cooking its own soup defence wise, equipment will be (and stay) outrageously expensive, as it is only produced in limited numbers. This will require a unified EU defence policy, and ultimately should lead to a common military. I wouldn’t centralise it too much, though, as this adds a single point of failure. Maybe transitioning via turning the existing national militaries into the EU military’s regional commands could work. Likewise, even equipment produced under unified standards shouldn’t be made all in one place, but needs to be spread out somewhat across the EU, as a single factory is way too easy to put out of production. Also, this way, it’s possible that every member state gets to benefit economically from the defence industry.

For nuclear doctrine, firstly, there needs to be sufficient capability. Unfortunately that means enlarging the nuclear arsenal. Currently, the EU’s only nuclear deterrent is the French nuclear arsenal, which was designed to have the ability to deliver a retaliatory strike powerful enough to make a single nuclear superpower think twice before attacking France itself, because they risk losing all their major cities over it. Now there unfortunately are three nuclear superpowers with potentially hostile intents towards the EU. In the remainder of NATO, there is another nuclear arsenal, that of the UK, which was designed around the same idea as the French one, and is sized similarly, but unfortunately, relies on US made delivery systems with shared maintenance arrangements, (Trident rather than domestically produced SLBMs) so it useless in the long term as a deterrent against an increasingly hostile USA, because they can render the missiles inoperable by ceasing delivery of spare parts. The British warheads also were developed in close cooperation with the US, so there might be a possibility of some spare parts also relying on US supply chains.