After having completed 0 and both Kiwamis, I can agree with you about Kiwami 2 is definitely on the lower tier of Yakuza games, given the Jingweon make for bland antagonists and the multiple plot twists thrown onto the finale got a little ridiculous (Terada somehow perfectly staged his shooting without any real issue? Even considering the Jingweon are expert assassins, that's a bit too convenient) and the Majima Saga was overall mediocre. I'd disagree on how fantastic the Dragon Engine is though as, despite its pros such as being able to ragdoll enemies, easily switch between weapons, etc. there's noticeable flaws such as how enemy guards actually punish you from trying to do skillful combos and instead encourage cheap spamming instead. Looking at comparisons to the original, it's also apparent how much of the narrative's atmosphere was diminished through changes to visuals and soundtrack (especially when the cuts cause some parts of the story to grind to a screeching halt, most of Chapter 13 in particular).
With Kiwami, I'd agree that that one addition did damage the story's pacing and Kiryu's character somewhat (if referring to that part with Majima in the main story) and some of the attempts to reuse 0's fighting style on enemies can diminish them somewhat (eg. Shimano just having Mr. Shakedown's moveset without the raw power that made Shakedown a threat) and the game being limited by the content of the original essentially does make it half of the game 0 was. The additions of Nishiki's development really did enhance the plot though and the Majima Everywhere system can lead to some great moments of gameplay, plus the game turned one of the few bad parts of 0 (The RNG Catfight mini game) into something that had some strategy to it (Mesuking).
As for 0, whilst I obviously have yet to play 4 or 5, I'd say some characters still definitely stand on their own as being very fleshed out or memorable (Kuze, Awano, etc.). I can see why playing it before 0 would dampen the conflict in the Saejima plot somewhat, though not to a damaging affect when the conflict still comes from Saejima's anger being fuelled by a misconception, plus the advantage of playing 0 first would be how it truly makes the audience care about Nishiki's later development. I can see the argument for Majima's focus being a bit forced, though it is in a different era and if there was any significant character that could do with more fleshing out through a prequel in the series, it's Majima as what was revealed in 4 still leaves quite a gap in his journey to being the Mad Dog.
All that said though, what idea of a Yakuza spinoff do you think would have more potential - A Majima spinoff set in the 10 years Kiryu was in prison, a Kuze spinoff set just after he leaves his boxing career for the Dojima Family or a Kazama spinoff centred around his life as a Dojima hitman?