This is the TLDR version, if you can believe that. I had a bit of an essay written out, giving the backstory and explaining things in a lot more detail. I can post it in a comment, or edit this to include it, if requested.

My wife and I (M) want to create a new relationship with an extremely close, long-term friend (F). We aren't 'unicorn-hunters', as we were not actively searching for this. The cards all just fell into place slowly over the course of several years, and we're right on the cusp of having this conversation with this friend, being about 99% sure she'd say yes.

Note that I'm saying 'create a new relationship with' and not 'invite her into our relationship'. We want this to be as even and fair as possible. We are willing to uproot everything, but we understand that we will always have a bit of 'couple's privilege', such as:

• currently married, although divorce is an option to help level the playing field.
• Shared history - 12 years married (although they were friends before we got together [15 years], we've actually 'been together')
• We have a child together (4 YO)
• Families likely won't like the idea of this new relationship and might be hostile to her, etc (our families both adore her as our friend, but it'll be interesting to see how that changes with this new relationship)
• Religious social environment (small town, deep south) and several religious friends. We'd all likely be ostracised, but she'd likely be blamed for 'leading us astray', or some such bullshit. She'll take the brunt of it, not that we wouldn't defend her.

What are some…pointers, I guess, from people who have been in this position? We know that obviously there will be things that she will want to talk about that we could not pre-plan for, but we want to not be blindsided by anything obvious, like we did not think through something that could be a potential deal breaker that we should have known about, if that makes sense. We don't want to get anyone's hopes up and then have to dash them, or just be downright insulting because we did not consider her in an extremely important way.

Things we have listed out so far, but we know there are things we have not thought of:

• Be up front with the relationship publicly as soon as possible. If that means cutting some family members or friends out our lives because they cannot handle that, then so be it. We've done it before (for other, nonrelated reasons) so we know we can handle that
• Get divorced to help level the playing field, and then have a (as public as possible) ceremony with the 3 of us
• Update power of attorney, for both of us and our child
• Update all school documents for child so that she can pick up kid, be involved in teacher's meetings, etc.
• Anything else required to have her serve as a guardian to our child
• Move into new home together (would need to anyways, as neither of our houses have enough rooms to accommodate extra people, nor do our leases allow that)
• Update all finances as necessary (potentially dangerous, I know, but generally shows how serious we are)
• Update wills and other legal documents.
• Make sure she knows that we won't 'veto' anything, and that she has just as much decision making rights/responsibility as we do. IE; we won't ever just drop her. She'll be in control of what happens to her
• Make sure she knows that if she chooses not to be with 1 of us down the line, she won't have to give up the other

I know these are all 'serious relationship' thoughts, and it might never go that far (if anywhere at all), but we want to show her that she won't be just our plaything. The last thing we want to do is hurt her (ourselves too, obviously). Does anyone have any advice for this? Most triad examples I see don't have the angle of the new person being a long term friend to both partners. Kind of throws a new spin on it.