“Security” connotes more than military defense. It’s often a concern of economy, which is secured through trade and partnership and international alliance.

You're not wrong, there are broader and narrower definitions of security. For the purposes of my posting, I used it in a more narrow, defensive meaning.

But tell me specifically the difference between becoming more independent vs isolationist.

Happy to. "Independent", in a security/defensive context, means that you can organise your own defense if need be. This does not mean that you're unwilling to cooperate. Like you said correctly, there are great benefits to be had from military alliances: joint development means reduced R&D and maintenance costs for everyone involved. Compatible command structures and shared strategies improve security for those inside and deterrence of those outside the alliance. But: if allegiances shift and your former partners drop out of the alliance, you're not opening yourself up to any vulnerabilities - unlike large parts of Europe, which has entirely relied on the U.S. for things like air defense, intelligence and nuclear deterrence, but also conventional land operations, and now needs to build up these capabilities in a rush.

"Isolationism", on the other hand, means you're flat-out unwilling to cooperate or care. You refuse to enter alliances, be bound by treaties of any kind, and generally take no interest in getting involved with the rest of the world, including the use of soft power.

The reason you're suspicious of the word "independence" and connect it to right-wing populism is because, like so many other words, right-wing populists love to abuse it: they say "independence" because it has a nicer ring to it, but in fact, they seek isolationism. And in doing so, they actually weaken their nations by passing up the benefits of cooperation. I dare not say whether that is up to irony or design - because they were bought by foreign powers looking to weaken the nation in question.

“Security” connotes more than military defense. It’s often a concern of economy, which is secured through trade and partnership and international alliance.

More spending doesn't automatically equal stronger. Especially if you're spending that money on highly complex equipment produced by an openly hostile third party, which can end its supply of spare parts at any moment and therefore render the capabilities purchased with that equipment all but useless.