My 2012 C5 was built in 2011, has the offset axle front fork with 18" front wheel, and shows some handling vagueness / mild wobble when it gets up over 68 mph. It's not a death shake, nor is it progressive, and I've pushed it through up to top speed of around 75 mph without the feel increasing or appreciably reducing.
I'm 6'1", 230lbs and disinclined to tuck. My machine was fitted with an oversized front sprocket by the previous owner. I think these items all work to reduce my machine's top speed.
I found that reducing front tire pressure by a few psi seemed to mitigate the wobble, but I don't do this, when I feel the wobble, I back off. I hardly ride this machine on expressways, anyway.
Prices on the 500 singles do seem to have hardened at a higher level since the model was "discontinued", or perhaps this is a pandemic related thing.
Around 4.5 - 5k seems to be the price at the moment:
https://chicago.craigslist.org/chc/mcy/d/chicago-rare-royal-enfield/7436757581.html