Your wise to play them slowly, better experience that way. I try to space the games out as much as my patience allows (I end up caving in and starting a Yakuza game every few months or so). Thankfully with 7 coming in January I can persuade myself to not replay Kenzan yet and just wait for that instead.
As for 100%ing the games... its a pretty big task for pretty much any game in the series, demands you learn certain mini game rules and do lots of different things in each game. Oh, and sometimes grind, some games are better than others in that regard though. I've 100%ed almost all of the series now, with just a couple left to replay for that honor, and something like Yakuza 5 I love 100%ing because while its obscenely long to complete, its a fantastic ride because it doesn't really feel like grinding, rather just doing everything the game has to offer, which I love. Contrast that with a game like Ishin which has some bloody boring requirements (talk to each pet type 500 times... yay...)
That said, even the most grindy game in the series to 100% is a more varied/fun 100% than something like Far Cry 4 - mechanically an excellent game but FC4 (for example) has a problem with repeating mission types for each new area, it doesn't vary very much later in the game, you'll basically be doing the same thing as you were back in the first couple of hours, only with better gear. Yakuza drip feeds activities and side stories and just lets you play at your own pace. Its wonderful.