The right play is to maintain relationships (including arms trading) with multiple major powers - as Canada's PM very deftly pointed out at Davos. Getting closer to China doesn't mean exchanging one master for another - it can and should be a way to increase the alternatives available, without going all the way in the other direction.
> The EU would also have opposed it if the US bought Russian, Chinese or Iranian weaponry.
This is such an implausible counter-factual that I can't even begin to imagine what would have actually happened. Still, I doubt any more than some "public letters" would have been issued, whereas I'm sure that the opposite would have resulted in actual economic pressure from the USA against the EU/NATO country that would have dared, under any administration.
I mean, you offered a basically similar, implausible counterfactual. I think we can agree that it is at least parties that the EU would have opposed purchases of Chinese, Russian or Iranian weapons by the USA and vice versa -- but Russia and Iran have been sanctioned for long periods of time (Iran, basically continuously) by both the EU and the USA, and Russia is the main territorial threat to the EU, so maybe only China is really an interesting possibility here.
Arms trading with China is probably not a good idea at all.