0 is definitely the place to start and the most solid standalone entry in the series, so I would start there and decide what to do afterward. But if I were to trim down the series to the “essentials” it would probably be something like 0, 2, 4, 6. I’d maybe toss 1 in there as well, since it’s short and introduces a lot of characters that are mainstays of the series.
Yakuza games aren’t quite open world, but they’re packed with lots of optional stuff to do, so an average play session can vary depending on how much you get into that stuff. There is however a lengthy and intense main story that you could focus all your attention on instead, if you wanted. But I think Yakuza is at its best when you at least mix in some of the side-stories, which open up gradually, and you are free to do them whenever there’s a break in the main story.
Combat is an action beat-em-up (at least until 7, which is a turn-based RPG) with a standard combo system and “heat actions” which are contextual super moves you can activate with enough meter. The main story will regularly throw scripted encounters at you, which are mostly large groups of enemies that you just plow through, and occasional a boss fight between cutscenes, which there are a lot of. There are also thugs roaming the city that will try to pick a fight if they see you run by.