I appreciate that you and Joey want to be seen as different, and you feel different. You are using unicorn-hunting language though, such as (in your ad):
looking for a woman to join our relationship
a lady who wants to ultimately become an equal in a thruple/triad
Perhaps you have not come across this article yet, despite being on the hunt for some time:
This article is a long, technical discussion of what being a "Unicorn Hunter" means, what's wrong with it, and how people in such a situation can try to avoid the negative stereotypes associated with such behavior and get the things that they are ultimately looking for. It is written in a…
davidlnoble.livejournal.com
That might give you some insight into why your hunt has been so long and fruitless, and ideas for how to to get what will be feasible and still meet your needs or desires.
Briefly, this mythical creature would not be "joining your relationship," "as a third," just like monogamy plus one. She comes with a lifetime of experience, her own wants and needs, perhaps already has a (poly) partner, or children, a home base, a career and family, roots in a certain place. She won't be magically able to join you on the road as a trucker, or move to whatever state suits you.
If she is very young and doesn't have these things yet, be careful. She may be needy and wanting a "mom and dad" arrangement.
She will never be a full equal, because you two are already legally married. If you two divorced, she might stand more of a chance.
If you two can be fully out as polyamorous, and introduce the unicorn to all of your friends and relatives, where she will be made to feel accepted and fully part of the group and family, welcome at casual events and holiday functions, and not looked at with suspicion, as a possible "cowgirl," "homewrecker," "side piece," etc., she might stand more of a chance for longevity and satisfaction.
If you are out as bi, and all your friends and relatives know you need to express that side of your authentic self, and support you in this, she might stand more of a chance.
If you can date her locally, normal dates, one-on-one dates with each of you, as well as dates as three, and take your time about fitting her into your lives, and her fitting you both (as individuals) into hers, before anyone feels rushed to move in together, she might stand more of a chance.
If she only really clicks with one of you, and can continue to date that one while remaining friends with the other, instead of being vetoed, she might feel safer dating (plural) you, at least to start.
If you do all get along like gangbusters, and dating goes well for a least a year, if not two, she will need her own room in your shared home, and freedom to add her personality to the rest of the house, as well.
There must be no envy, no jealousy, if one of the dyads develops at a quicker pace.
dyads:
You and Joey
You and Unicorn
Joey and Unicorn
metamours:
The same. Everyone is everyone else's partner AND metamour. This is very hard to navigate.
Maybe the Unicorn, if she is an established 40-something with a full life (like yourselves) will be very busy with her child(ren), her career, eldercare, hobbies, a home, friends, etc... She is an actual person. She deserves space to live and grow, as well as space to love.
One more factor is that a woman who wants to date another woman might be a full-on lesbian and not want a man anywhere in the mix. And sometimes a unicorn will act like she's bi, when she really only wants the man of the couple, and only gives lip service (pun intended) to the female partner at first. Or, if she's secretly a lot more into women, she might only begrudgingly include the man (as a "stunt cock," say) while giving most of her romantic attention to the woman.
For firsthand triad journeys, I'd suggest reading the blogs of:
SquishyHusk
https://polyamory.com/threads/trying-out-a-triad.155709/
and Kynde
https://polyamory.com/threads/homesteading-triad-dynamics-and-the-family-calendar.156375/
SquishyHusk is a man in a triad where the women are more romantically attached and he always feels kind of left out. Kynde is a woman who was in a triad where everything seemed perfect... until it didn't.