Yeah, 0 is a prequel and probably the best game in the series. It directly precedes what happens in Kiwami (about which you're right, it might not feel as good as 0 but it's still very good). A great place to start. The games are interconnected but 6 puts most of the classic characters in the background and focuses on Kiryu and Haruka, with some bits of Akiyama and Date and mere glimpses of Majima and Saejima. It also uses a new engine so it'll different, in some ways better but in some ways worse. It's also worse than Kiwami 2 which is a remake of Yakuza 2
Chronologically it would go 0, K1, K2 and 6 but K1 might feel like a stepback
from 0 (but not necessarily) and 6 might fell like a stepback from K2.
Going 0, K1, 6 and K2 might be the best option IMO. Two games on the old engine and two games on the new engine. Also, you don't need K2 that much to understand what happens in 6, the stories of both games are a bit more standalone.
I think you'll like 0, the series is very unique, fun and oldschool. Maybe just as charming and unique as Nier but more accessible and slightly less weird
Japanese voice acting is the only option in the Western release but I also want Japanese subs which can't be selected unfortunately:/