Sput, they've pretty much covered everything above, but yes, the games are absolutely pure Yakuza-vibe and have all of the same quirks that the rest of the series has:
Kenzan! has multiple fighting types, which was the series' first foray into multiple styles and yes, while the characters are reused, there are only 3 that are prominently reused (take a guess which ones lol). It was, as mentioned, the very first PS3 RGG title and there are some frustrations, but otherwise, it is solid from top to bottom, has a great plot and this was my first exposure to Zeebra... which led to more exposure to Japanese music and I discovered just how awesome it is. It is indeed a loose retelling of Miyamoto Musashi
https://en.wikipedia.org/wiki/Miyamoto_Musashi and it takes place in the Edo period, soon after the Tokugawa clan rose to power and prominence.
Ishin! also has the multiple fighting types (done more effectively than Kenzan!) and introduces some of the gameplay quirks that 0 has (such as pressing a button to run, among other things). The Another Life involves farming and cooking and the game is just chock full of content; until 7 came out, 5 and Ishin! were far and away the most content-heavy games in the series. As mentioned, it takes place in the Bakumatsu period, essentially culminating around the time that Commodore Perry landed in Japan (which takes place near the end of the game) and it involves Sakamoto Ryoma
https://en.wikipedia.org/wiki/Sakamoto_Ryōma, the Shinsengumi
https://en.wikipedia.org/wiki/Shinsengumi and Hajime Saito
https://en.wikipedia.org/wiki/Saitō_Hajime, the Meihi Restoration. Where Kenzan! had a few characters from the series until that point, Ishin! VASTLY expands on that. After 5 came out, there was a, "character election," whereupon fans could vote for their favourite characters from the series, to be used in an upcoming game. This was a major deal in 2013, so people voted like mad for 30 days (and, due to a loophole discovered around day 10 of the election, the, "max 10 {or 3} votes a day," could be exploited, thus you could vote *unlimited* times per day... and I did lol) and the top 20 characters or so, were used in some capacity in the game... but there are more than 20 who are represented, you just need to find them all

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Both are great games, but Ishin! is one of the series' elite titles by far and it is definitely worth a playthrough, even if you need to translate it.
On the election note, for 0, they also did that with the JAV actresses that appear in-game and the top 10 were used as the Cabaret chicks, Marina and a few more prominent ones. The next 30 or so were the ones used for the videos and phonecards.
You could not exploit the voting in this one though, sadly

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