What's worse: Americans keeping their hands out of another countries business or Americans involving themselves in a war on behalf of a non-ally, then losing?

If you want to engage in ruthless geopolitics, then the second can be superior. If you believe that Russia is your enemy (which it's not, America made it one), and put weakening Russia above everything else, then it certainly can be worth it.

China really only cares abouy Taiwan because they think America could use it as a forward base to interfere in their domestic affairs. Do you think they'll be reassured by the US fighting a proxy war against Russia through Ukraine?

Yeah, I find it rather unlikely that China will be deterred. The best way to deter China, in my view at least, is to not worsen the situation for China. Shipping massive amounts of weapons to Taiwan will make it a now or never like it did for Russia.

They have veteran forces where Ukraine was newly trained greenhorns

I actually disagree. Ukraine has had 800,000 men spending the past 9 years terrorizing the Donbas. In total, not any given year. So there are a lot of men with military experience, though mostly shelling defenseless civilians. Russia has had low-footprint wars in Syria, and a pseudo-war in Chechnya against non-entities. So Ukraine definitely gets the + there.