Minimony attire

What about semi formal attire? Or is it casual dressy attire that you’re looking for?

I agree with pp. dressy casual sounds good

Totally agree with the others- semi formal would be great.

I googled, "cocktail attire brunch," hoping there was a word for that, but apparently there is not. Since you're having a limited guest list, I'd just use outfit descriptions for anyone who asks.
I can imagine myself saying:
"Probably a knee length-ish dress, like something you'd wear to a really formal baby shower. The guys should wear ties, but they don't need jackets."

Or: "If I was a guest I'd probably wear that short navy dress that I wore to J and H's rehearsal dinner, but with more jewelry and dressier shoes. Your husband should wear a suit, but he doesn't need a tie."
Alternatively, you could just have them wear whatever they would've worn to your originally planned wedding. At our backyard miniwedding, almost everyone was in either a suit or a long dress. As long as everyone is dressed in a similar level of formality, no one will look out of place.

Ah yes! Dressy casual sounds perfect. My mom wants FH to try on a tux they have and I’m having a hard time letting her know that is far too formal for this event lol thanks everyone!!

I think it is okay to say Sunday best, if that is what you want. How many people are attending? If it is not that many people or households, I'd just text them all individually relaying that information, but our minimony guests are basically immediate family and bridal party so we were comfortable telling them about exact attire.

Smart Casual?

I just wore a gown in my favorite color and he wore a suit.