Trump isn't worth listening to.
Europe is a special case. They've been at war with each other for millennia. There are constantly shifting interest groups and alliances and belief systems.
I did some low level legal work for a NATO reserve organization and it quickly became apparent that Western Europe exists in two blocks: the Nordic block (which includes the UK) and the Latin block (which includes France, Italy and most things with Roman roots. The northerners are practical and business focused the southerners more laid back and independent thinkers. If you want to build a consensus amongst Europeans then you have to be able to leverage what motivates each side. There's a lot of give and take there - you have to feel your way around for the red lines which a given side won't cross and find the positions that have been put up as bargaining chips that can and will be traded away.
I left out the Americans. They were there too and in short, they're the bull in the china shop. They ain't subtle and usually leave frustrated very quickly. It takes a very special American to feel his way through the hurdles. At my level they didn't exist but at the higher levels, occasionally you get a good one.
I can't say that the Obama administration was particulalry good because their aim for improved defence wasn't strong enough and their powers of persuasion weak. There was little post 2014 leadership that got any traction. Trump is a bully and if there is one thing the two European factions can agree on its that American bullies are assholes and won't be listened to; heels get dug in all around. It did get them to start turning more to each other because dependence on the US was looking tenuous. Biden - well. What can I say. It was Biden and I'm not so sure he understood what his advisors were actually doing. It reinforced the fact to the Europeans that they needed to turn more to each other. The "second coming" is a bit of a shock to everyone - IMHO because it shows just how desperate and polarized the American public is. I don't think that anyone, including in the US administration, has a clue from day-to-day where things are heading next. Uncertainty is never a good thing in world politics and there are nasty actors out there just itching to take advantage of it.
When you say if they'd listened to him in 2018. The problem is when there is a constant stream of drivel coming out of his mouth when do you say "Eureka! That one's a golden nugget!" and hang your own policies on it. IMHO, European defence policies are maturing to relying on their own resources. They have a long way to go. As have we.