Since the Pledge combos do "generate" their respective Field Effects, they can be extended by the Amplifield Rock (provided it's held by the second pokemon in the combo). This should help make that method a bit more reliable, right?
Speaking of reliably setting up the Pledge combos, here's a set that'll help get them going more reliably in Single Battles:
"Vibrant Pledgemaster"
Note that you should keep the Pledges on your delphox and primarina if you grab this, since it's always nice to have a backup plan in case something goes wrong.
Delphox: I recommend slipping Grass Knot on this one as soon as possible. That will give it a Super Effective coverage option against the many Rock, Ground, and Water Types that threaten it's lifespan. The TM is admittedly a bit far into the game, and you have both of the other parts of the FWG Core already, but what's wrong with being a little more self-sufficient? If that doesn't suit your fancy, you could go for Dazzling Gleam instead- it's only a little farther in the game, and it lets delphox blast enemy Dark Types with an attack that gets boosted in both Rainbow Field and Misty Terrain.
Primarina: This one's a bit an odd one. On one hand, having more than one way to twist the battlefield to your advantage is a sound idea, and Misty terrain isn't too dissimilar to Rainbow Field to begin with. On the other hand, though, the two are different enough that a lot of your pokemon won't be able to take quite as much advantage of the mist as they would the rainbow, so it'll probably be best used in situations where you need Misty Terrain's most significant differences to claim the victory. If you decide that it just isn't able to pull its own weight most of the time, you do have the option of replacing Misty Terrain with Psychic- this will let it strike at opposing Poison Types that seek to take advantage of its Fairy weaknesses, though it'll still have to rely on others to deal with opposing Steels anyway.
Roserade: Hidden Powers aren't able to be manipulated, at least not at the current point in time. I'd still keep Technician as the Ability since the Factory Field continues to be ridiculously common even into the lategame, and it's not like roserade's other Abilities are all too impressive anyway. If you ever find yourself feeling like Leaf Storm's Sp. Atk. drops are too much of a hindrance to work with, you could probably get away with replacing it with Magical Leaf- it always gets the Technician boost, benefits from Misty Terrain, and can't miss under normal circumstances, making for a fairly stable option. Another option would be to trade something out for Nature Power. It turns into Aurora Beam in the Rainbow Field, and while it does miss out on the Technician boost (65 BP moves are so annoying...), it does allow roserade to function as a Field-boosted Dragon-and-Birdslayer with a 20% to drop their Attack stat if they survive. It also just helps with coverage on other Fields in general (in case you find yourself unable to set up your own for whatever reason), which is a potent tool on its own accord.
Yanmega: This one's got an interesting option: Silver Wind's* power is boosted on Rainbow Field and its omni-boost chance is doubled to a hearty 20%. It's still a bit weaker than Bug Buzz, but being far more usable than it was across the board makes it an interesting option. Yanmega in general has an issue with being hard-walled by Steel Types, but picking up a couple boosts along the way would certainly help; if you decide to take this path, trading Speed Boost for Tinted Lens (double damage to NVE attacks) might be a good idea. If not, then Giga Drain is probably the more useful of the moves you mentioned, I think.
*Silver Wind is an Egg Move for yanmega
Weavile: This one... just seems a touch lacking in options? I guess Fake out could be used for the last slot because, well, it's Fake Out, but eh. It doesn't have any means of taking advantage of the Field Effect, and the Magical Seed its wasted on it, so those shenanigans aren't exactly the wisest course of action either...
Aurora Veil: In a vacuum, this move would certainly be worth it. Setup-free, one round Dual Screens? Yes please! The issue is that the small pool of pokemon that get it aren't exactly the most useful for you. Alolatales' Snow Warning both actively hinders the party with chip damage and destroys the Rainbow Field, and its other Ability useless without Hail (thus making into a primarina role-wise). Alolaslash and glaceon are also Ability-less without the the Hail, mew and articuno are unavailable, and both froslass and delibird are uninspiringly frail as Veil setters. With jynx and cryogonal sharing the flaw of glaringly low Physical bulk and aurorus having a bizarre inability to even learn Aurora Veil in the first place, I just can't bring myself to recommend it this time. I guess glaceon at least has the advantage of also having access to the buffed Wish?
Altaria: I wouldn't suggest running the Mega for this one, at least not for anything more than just using Mega Altaria normally. The Field Notes specify that post-Normalize attacks benefit from the buff to Normal attacks, which heavily suggests that it looks at the moves Type after its been filtered by the Ability, so a Pixilated Hyper Voice wouldn't get the damage buff. Honestly, I'm a bit surprised that the cloudbirb doesn't have more ways to use the Rainbow Field- it seems like it'd be thematically fitting to me. At least Mega Altaria is a good 'mon on its own, right?
Hopefully my long and drawn-out thoughts will be some use to you, and good luck on your colorful endeavors!