Bridesmaid colors & guest attire for a champagne & ivory wedding dress?

You don't tell your guests what attire to wear. This style is often referred to as "boho" too. I think your girls could still wear champagne to compliment your dress. Blush would look nice too.

I think champagne bridesmaid dresses would still be okay! Blush would be a great back-up.

"Garden formal" sounds great for the guests. I would just go ahead and put examples of what this means to you on the wedding website or the invite since guests won't otherwise know what you mean. My wedding website is through Zola and they have a bunch of common attire suggestions - "Semi-Formal: Tuxes and gowns are welcome, and so are suits and cocktail dresses." / "Garden Attire: Wear sundresses, floral prints, suits in lighter hues, and fun hats and fascinators."

Neither of those are quite on point for you, but I think it's fine to spell out what a dress code means. I would keep it more general, rather than telling women they need to wear long-sleeved lace dresses (which might be your vision, but is asking too much of your guests). Suggesting that they wear lighter colors, for example, is fine though!

We almost went with beige/tan bridesmaid dresses, too! Instead, our girls are wearing dusty sage (not Azazie dusty sage) -- it's a grey-ish version of sage green, and it looks beautiful with all of the greenery in our wedding. Maybe something along those lines, or maybe a dusty steel blue?

As for attire, we noted ours as "cocktail attire" -- personally, as a guest, I like knowing what style the wedding is before I pick out my outfit (I wouldn't wear a cocktail dress and stilettos to an informal wedding, but would instead wear a sundress and dressy sandals).

I agree - OP, you unfortunately can't tell your guests how to dress. I WISH you could! Haha. You can, however, have a note about dress code on your website, or if a guest asks you can make suggestions.

As for your BMs, a lot of brides like azazie for style and color selection, as well as price. THey have a pretty champagne or frost color. I also think your idea of blush, champagne, or muted colors / earth tones for their dresses would be beautiful as well.