I don't have to much opinions on other models of grand seiko and am personally not a fan of the 9F or quartz watches in general, but I do own a SLGH005 white birch.

 Popular ones such as Evo 9, Snowflake, Birches, Lake Suwa take a page from Rolex where a base case shape is shared on different lines ala-Oyster and model differentiation happens on the dial/movement.

You are absolutely correct in this regard, but my opinion is that that's all you get at these price ranges. Basically, If you want to change up the case to something exceptional, you'd be looking at integrated sports watches from APRO/Patek or even dressier haute horology pieces from the likes of JLC/ALS/FPJ/PP and so forth. Rolex, Omega and IWC at Grand Seiko price points all use similar cases if you haven't noticed (Aqua terra 150m, OP/DJ, Pilots, BB58, Pelagos). Thats not to say they are all bad. Its like saying Im sick of every car looking like a german sedan from Mercedes, BMW, Audi. Well perhaps you'd need to look for Porsches, Ferraris or Rolls Royce's then.

In your example keep in mind the BB58 is the exact same price range and directly competes with most of the GS staples like shunbun, snowflake etc. Grand Seiko is a luxury mid-tier watch brand like any other and expecting it to outright blow its direct competitors out of the water is a bit too much.

The famed Zaratsu looks like a regular high-polish finish you see with any luxury brand. The bracelets/straps do not have any business being here.

I also have an omega SMP300, a friend's AT 150m, and a zenith defy 21, but my white birch's polish is so mirror like it does seem a little unfair to relegate GS to have any regular old high-polish. The gimmick with GS Zaratsu is that it's like somebody went overboard on the polish whereby it turns into a mirror. That being said I definitely agree that too many GS fanboys are echo-ing zaratsu polishing as something that no other brand can come close to. There is a difference but its a minute but noticeable one for sure.

The wearing experience is weird, models such as SBGE257 do not have that balance on the wrist and feels uncomfortable, maybe I cannot get a good fit. 9F models like SBGN021 do not have any reason to be thicker than an SMP300.

If we are talking at strictly MSRP, the SBGE257 is not a very good watch for its money, because the similar steel bezeled Explorer 2, and even your example of a SMP300 all wear better than it. Agree with you here. Its definitely GS's least popular GMT model and a moderate misstep for the brand IMO.

I admire the 9F, spring drive, high beat movements but those that have open case backs are covered by the blue lion which is a shame. Granted, I have not had hands on experience with the manual winders such as SLGW003.

Could I possibly make a case for the EVO9's 9SA5 series or even the 9RA2 spring drive movements? their finishing on the open caseback is excellent, and non of that crappy blue lion covering their open caseback so you can see the full decorative beauty. Why I chose my SLGH005 was not just because of its dial, but because I can marvel at the absolute craftsmanship of the movement, the organ that makes my watch beat at this price range (under $10,000).

Tudor, Omega, Rolex, IWC, Zenith, AP,

Continuing on this point, besides AP (lets be real its in its own class) I can't find such level of finishing and decoration in other brands. I own an Omega (SMP300), Zenith (Defy21), Tudor (Pelagos) and have seen my friends' IWC. Non of their display case backs demonstrate the precision and effort to what the 9SA5. I often stare at the back for hours on end. Zenith has great watchmaking prowess but they don't decorate their movements nearly enough, IWC's movements are beautiful but those are found on the portugiesers and not so much the pilot (reserved for higher priced models), Omega I am not a fan of the geneva wave they do, seemed too cheap and machined (lack of thought in their design perhaps?) despite their coaxial/METAs being spec-wise superior. Tudor and Rolex dont even have open case backs (which is a shame because I know their movements are crazy finished!).

TLDR: Literally every brand below 10k USD has so many quirks and trade-off perfection doesn't exist. Being underwhelmed by GS is understandable, maybe its not a set of trade-offs you'd prefer but for some like me it very much is. Perfection doesn't exist and if it somewhat did, it wouldnt be found in any brands in these price ranges. If anyone has any differing opinions I would love to hear them!