So I was up last night and curious about how things improve in game design, ui design, combat design, graphics-design as the series progressed from Y3 onwards. Watched some videos and read reviews of all the games that came after that. My takeaway is that Y4 is basically just Y3 repackaged with 4 main characters and everything that's weak about Y3 will be weak in Y4 except combat should be a bit better with the 4 characters.
Then it sounds like Y5 improves with a new engine. So improves the graphics, improves the UI (quicker into/out of random street battles, thank god I like the wind-up intros for bosses or story stuff but there's so many goddamn street encounters [encounter rate is way too high in Y3 on the street] and the animations are zzz), adds lots of new locations and new gameplay ideas and does a bit more with the Y4 characters and combat. It seems super lengthy and bloated, but a strong improvement over Y3/Y4. I think I read they start voicing more of the game from Y5 onwards.
Then Y0 sounds hot shit, a bit shorter/smaller, but refinement of Y5's UI improvements and graphics & engine, good story/pacing.
Then Y6 comes in with a new engine again, this time combat is changed up and supposed to be pretty awesome, nice graphics, no loads, fully voiced. Sounds good.
And Kiwami 1 basically just sounds like Y0 with RGG1 story, and Kiwami 2 is refinement of Y6 engine/improvements with RGG2 story.
Coming from all that...I'm not going to play Y0 next. Sorry Cindi. I think going back to Y4 after Y0 would be pretty bad and frustrating to get through. It sounds like at the end of the day now that RGG1&2 have been remade with more modern gameplay, that 3&4 are gameplay/ui/graphics/presentation-wise now the low parts of the series. So I think it's better for me to get through Y3 and then play Y4 (and probably play Y5) before Y0 and then Y6. I think playing them in the order released will let me appreciate the improvements the team keeps making year after year.
Right now I'm not interested in playing Kiwami 1&2 because I already played those stories. But after I finish Y6 (and Fist of the North Star Yakuza) and there's no "new" stories I'll probably go back and play Kiwami 1&2 which should both hold up fine a couple years down the line.
Apparently Y4 PS4 is coming in the fall (no release date yet, so probably late fall), and Y5 PS4 in Spring 2019 (so then I'd play Y0 next summer, Y6 in the fall), so that sounds like a good spacing to play the series. Trying to play these games more than one every 3-6 months is easy burnout.