So what's the solution ? Assign a surveillance UAV to every Russian ship parked "without a good reason" over a cable ? It would be expensive, but doable, and create a reserve of vehicles for wartime use.
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Why did Putin take crimea under Obama's watch, parts of Ukraine under Biden's watch, but then not make any huge moves like those while his "asset" was in the white house?
Also, he needed a green light. Which was provided in the form of the chaotic Afghanistan pullout in 2021. Not that he was counting on it -- but once it went through, it seems very likely that tipped the scales in his mind in favor of deciding to actually go through with the full-scale invasion in 2022.
Imagine how quickly ukriane would have collapsed if the US was not providing support, and the US was preventing European nations from providing support. And then imagine how well off Russia would be if there were no sanctions placed on it by America. All in all your point doesn't make sense. You don't get an asset sitting in the oval office and then not use them.
You may want to think about the chronology again.
And then ask yourself if your statement above still makes sense.
I'll leave out whether this was intentional or because he saw peace in putin's eyes (as he himself claimed).
But yes, russia couldn't successfully invade in 2018.