In all reality, if you are looking for the utmost in reliability and quality, you'd probably be better off with one of the Japanese manufacturers. If you're willing to let that take a back seat towards getting a 401 with some soul, style, and uniqueness, then quibbling over some minor differences regarding perceived quality differences between the 18/19/20 model years shouldn't be a deciding factor. Rather, I'd go for an individual bike of any year that you feel has been well maintained, has service records, and you get a good vibe from the owner. I'd rather get a bike from a responsible rider than someone who has been stunting and thrashing on their bike, for example. I've got 3200 trouble free miles on my 2020 so far, so I have no regrets about getting one with "lower quality".
Note, for 2020, the first valve clearance adjustment isn't required until 9300 miles. For the 18/19's, it was at 621 miles. Also, you'll have more factory warranty remaining with a 2020 (2 years from purchase date) compared to a 2018/2019.
So, consider what is most important to you and what gives you the most peace of mind when it comes to buying a used bike.