Welcome to Natdex OU, I just wanted to give some points on your post.
While Specs Charizard is tremendously powerful, it is weak to stealth rocks as you mentioned and relatively frail. Even if you go the Heavy-Duty Boots route you still lose out on Mega Charizard Y, which can be a great additional sun setter.Specs Charizard is nothing to underestimate, 2HKOing even Blissey, the mighty Special wall. With Flamethrower! One free switch in, and you will unleash all sorts of destruction unless the opponent has specific answers, all of which are vulnerable to Charizard's coverage.
I'd say Gliscor is outclassed by Great Tusk, which gets the protosynthesis boost in Sun and matches the fast pace Sun teams like operating at better. It's also why I think Mega Sableye and Xatu have no real place on Sun, they're more suited for drawn out games.Additionally, partners like Gliscor who can Defog are natural partners who can appreciate the opposing water types getting removed by combined assault. Magic Bouncers from Espeon, to Mega Sableye, to Mega Diancie and even Xatu are great as well.
Flutter Mane has been banned to Natdex Ubers.I haven't even explored Protosynthesis mons like Flutter Mane, who will add a whole new level of destruction.
I don't really see why to run Flash Cannon over a move like Surf, considering all the Fairy types are unable to stand up to rain boosted water attacks anyways. The only exception being Tapu Fini which is neutral to Steel anyways. Similarly I don't understand why you'd run tera Steel over tera Water. The extra power on water moves is greatly appreciated and I don't see against which Pokemon the Steel tera type would come in handy defensively.Hydro Pump and Draco Meteor alone are very powerful, but when given coverage like Ice Beam, Hurricane and Flash Cannon it can break past things that would try to stand up to repeated hits. Terrastallizing into a Steel Type is by far one of the best things you can do with Kingdra, as it has the movepool to utterly drown anything that would dare try to attack it.
Rain stall has basically stopped existing once weather was no longer permanent. Similarly to how Mega Sableye, Gliscor and Xatu don't fit on sun, rain likes to move at a very fast pace as well. A rain stall team would have to switch far too often to its rain setter or use rain dance over and over again, which is very exploitable. Ferrothorn does still have a place on rain however because it's not really a passive Pokemon, it can do quite a bit of damage and support the team with hazards.As somebody who has played Gen V's OU, I know that Rain Stall is a thing, even with the nerf to weather from Gen VI onwards.
I definitely agree rain is a lot stronger than sun, it will be interesting to see how its viability will shift with the recent bans.So far just gonna keep experimenting with both weathers. I think rain is stronger overall than sun as it requires less support, but I don't know if it is banworthy yet. Open to seeing how the meta develops.