You are demonstrating a severe lack of understanding about how nuclear weapons, doctrine, and policy, work.
First off, there is a difference between tactical and strategic nuclear weapons. Tactical weapons can be deployed by something as small as an MLRS, and be an alternate payload for a strike fighter, missile cell on a ship, or even used as a glorified remote-detonation mine.
Strategic weapons are the big bombs and missiles that can generally be detected from long ranges, and are what provoke immediate responses when used.
Second off, there are only two nations in the world with currently-relevant systems regarding intercepting missiles, the USA and Israel. If there actually is a strategic nuclear exchange between the USA and other major powers, the USA is the only nation with a chance of coming out partially intact. The exact odds are unknown, but there is at least a chance, and with the current development arc of technology, the US's capabilities in that regard are going to be improving steadily, while it'll be decades or never before anyone else has the capability.
On top of that, given the US has a habit of developing technology to counter what it itself is already doing, odds are decent that by the time anyone else comes up with said capabilities, American penetration aids and the like will already be designed to overcome them. Not guaranteed, but a solid chance.
Third off, responding to tactical nuclear weapons with all-out strategic launch is standing policy for nobody.
If a hostile power tries to send their military across the Atlantic or Pacific, and the US responds with tactical nuclear weapons that blow the shit out of said invasion fleets, this is not an automatic 'and now everyone launches nukes and everyone dies.' It's also important to note that due to shorter travel distances and different delivery systems, tactical nukes will generally have detonated before there's even the chance at a strategic response, meaning decision-makers will have to make said decisions after the fact, not when tactical weapons are in the process of closing on their fleet.
Further, the very mutually-assured destruction you are talking about actually makes it less likely that tactical use will result in a strategic response. If the US tac-nukes the fleets, the enemy nations are out their expeditionary forces, but the nations themselves are still fully intact. If they then reply with strategic weapons, the US does likewise, and then everybody in major cities dies.
This means that MAD actually works against the invaders, because they can either accept the loss of a big chunk of their military, or they can all die in exchange for killing tens of millions of Americans out of sheer spite. They gain nothing except spiting America by launching strategic weapons.
The same is largely true for any other nuclear power. It is generally understood that attempting a conventional invasion of a nuclear power invites defensive nuclear response, which is why it'd be suidicing your military for little to no gain. India and Pakistan are something of an exception to this with each other.
The problem lies in the fact that all of this goes into the trash when America goes into isolation.
If any power becomes a superpower in a post-America world. It will make the rules, not the US. Americans will have to adapt if this superpower sets the rule, a nuclear strike is a nuclear strike and any attempt to use nuclear weapons on anything belonging to them is met with a full response.
It is the Americans who will be forced not to use the atom because it very quickly turns into destroying themselves and accomplishing nothing. What's the use of the atom in defense if you're going to die yourself right after? In the case of a simple defeat, you always have the opportunity to rise from defeat and get even later. After nuclear annihilation, there will be nothing like that. You have to be mad to use nuclear weapons.
To put it shorter sets the ball rolling on the US side, you want to defend yourself? Use something else, the nuclear solution immediately goes right to the top of the escalation.
And assuming that America will have a technological advantage in a situation of isolation over a superpower that just dominates the world. Particularly since isolationist America doesn't have that strong reason to keep its overgrown arms industry and research in its new situation. This is a heavily naive scenario.
A very strongly naive scenario, why should a country that is primarily concerned with itself still have an advantage in anything over a country that has just emerged victorious from a struggle for world domination. Such a state has already faced an analogous scenario with another country and won. They wouldn't have thrown themselves at America if they didn't see it as an opportunity to win. If someone already attacks America on its soil, they can probably handle even tactical nuclear weapons without a problem.
For them, America is just the ultimate boss to kill. The former hegemon who is gone but we want him never to return.
Fortunately, this scenario is as real as America's isolationism. That is, it is little, but it is possible.
Also, no one is going to seriously invade the US; the CCP or Russia might try something stupid in Alaska, but the US can keep our CONUS shores protected without a lot of foreign involvement or materials.
Today? Not for now, but in the future? Especially in isolationism? Most likely yes although it would be better if it never happened.
Look, you seem to be mistaking me for someone trying to push isolationism, rather than try to help make sure the US is less isolationist by pointing out when Euro's rant about what America needs to be/must be, it causes more isolationist feelings most of the time, not less.
Yes, people are tired, and getting their asses tired by melly lazy people only exacerbates this fatigue more. But this does not remove the problem that someone very much wishes to remove America into the shadows, and will exacerbate this fatigue for their own interests.
There will be someone if I was a superpower that wants to expand and my influence and a strong enemy bother me. And fighting him in his current state is too much trouble, if not suicide. I would strive to convince a hostile society that it is too tired, that allies leave them and parasitize their efforts. So that he himself, of his own free will, went into isolationism. That way I would have my hands free for my activities.
make other nations start paying what they said they would
There is a way, but you don't like it because of your anti-imperialism and liberalism. It is called zamordyzm (zamordism) in Polish. That is, literally taking someone by the face and forcing them to do what you want. If you don't want to play policeman, find volunteers, force the unwilling to help (you have enough power to do that) and bring the resistant down to the first floor and make them less important than they are and give the power over them to the volunteers to watch over them.
Otherwise, it's best for certain countries unwilling to help keep order *cough* Germany *cough* pull the plug and bill them for their past security services. Say you will pay so much and so much to maintain the US Army and US Navy to protect your area or they will keep order themselves but at their own expense. If the answer in both is no, then in response remove the protective umbrella over the country in question and thank you for your cooperation so far, for because it makes no sense.
That's what I'd do if I were you, weary of the constant wars to protect those who are shitting all over America and plotting behind their backs anyway. In short, give kick in ass to the parasites. They seem to have forgotten how things are.