I can certainly vouch for 4 Attacks' effectiveness as a cleaner, revenger and in fact sweeper especially VS more frail offensive teams. I run double STABs, Jump Kick, and Megahorn, though over the latter I am considering Wild Bolt to not be walled by Gengar etc. With SR, LO and Adamant its surprising how many things turn from 2HKOs to OHKOs. In any case, Megahorn allows OHKOs on the Latis, as well as Grass types, which can prove useful.4 Attack Set: This set's designed to destroy weakened opponents at endgame. Double-Edge/Jump Kick/Wild Bolt/Faint Attack. There should be no holes in this set, but 4-attack often ends up failing to KO targets.
Swords Dance: The most powerful set, but the one with the least coverage. The standard moves would probably be Double-Edge and NPow. The last move decides what you simply don't cover. Jump Kick takes out Heatran and Bliss while 2hkoing Natt but means no Skarm or Balloon Shandera/Heatran, Wood Horn beats bulky Waters and Rock and Subbing Erufuun but no means no Skarm, or balloon Shandera/Heatran, Wild Volt can 2hko Skarm, KO bulky waters and gyara, and nail Shandera/Heatran in the air but doesn't really KO any of Mebu's threats like Jump Kick and Wood Horn.
SD I want to try, but the issue is less losing out on coverage as Normal/Grass/Fight for example is pretty good, but that idk when you would find time to set up. Venu has quite a hard time getting a Growth if playing conservatively - the best target being Burungeru - and even then non-special sets risk being crippled by a burn. Mebu meanwhile has less bulk on both sides and is vulnerable to burn as well as toxic, and fighting moves. I think Wild Bolt as coverage has better utility in the final slot.
I definitely agree that 4atks and SD are its most viable sets. However, Cheer Up allows mixed to work if you need something hellishly fast, but like you said it does risk being outclassed.I've noticed that a lot of the Sun Sweepers have an amazing movepool. Problems with that is that you have to find something that that one Pokemon excels at.
Venusaur does the Mixed set better do to growths +2 boosts and similar coverage. Growth/Petal Dance/HP Ice(Fire)/Earthquake gives amazing coverage only resisted by bronzong as far as I know. HP fire allows you to OHKO Skarm and Nattorie, whereas Ice takes out the Dragons.
4 Attack set looks very viable, as it offers great coverage. Also the Higher base speed means that it can still function well enough out of sun.
SubSeed should be left to Erufuun/Jumpluff. Erufuun gets priority on it and Jumpluff has that nice base 115 speed to work with (Not to mention if it has Clorophyll.)
Swords Dance looks like it suffers from Moveslot Syndrome. You could drop NPow for Wood Horn/High Jump kick as they provide about the same type coverage.
Slowbro/Slowking work great on Sun Teams. Passing around Toxic allows for some key OHKO's sometimes, and regeneration makes it so that it can come in and wall stuff forever basically.
Has anyone else tried out Rhyperior on Sun teams? With the sun up it isn't OHKO'd by any water attack (Running Bulky EV's) and can 2HKO Crocune. Running Swords Dance turns 2HKO's into OHKO's with Rocks up. I use a dual set up set of Rock Polish/Swords Dance/Earthquake/Knock Down. Knock down puts Skarmory on the ground where it becomes death fodder for a earthquake.
Also SolarZard should almost always hold Lefties. It allows it to survive longer, and with its insane attack power it doesn't need any extra power from other items. Mine generally OHKO's everything anyways.
I think in terms of Slowbro/King I'd go for Bro to be less Pursuit weak. Toxic however really isn't that helpful to offensive Sun at least, since it can typically OHKO almost everything with a Growthed pokemon, but on Sun Stall could be pretty useful.
Rhyperior or Regirock I've used as Fire move absorbers given their resists. Grass being resisted by all Fires and Grasses helps him work well too. A Dual Dance set seems quite interesting of itself, what spread do you run, bulky or offensive? I think if I tried it I may forfeit RP for something to hit levitating ghosts and such, but its preference I suppose.