My primary partner and I broke up, but very much still love each other and are going to continue being in each other's lives. We're struggling what finding terminology to tell people who we are to each other. "Ex" is technically right but honestly wrong, "best friend" is true but not strong enough. I'm just in need of more relationship-describing words so I can get people to understand two people who love each other deeply, feel like family, are sharing a life, but aren't partners.
First off, huge kudos to you two for having the emotional intelligence and maturity to recognize that relationships grow and change and don’t always conform to existing, prescribed structures. Intellectual, emotional, domestic, sexual, creative, romantic, and other types of connections are up to us to define and one doesn’t need to include or proscribe another. I recommend checking out resources around Relationship Anarchy, which does a good job of giving words and structure to ideas and feelings like this.
Ultimately, the big issue here is whether it’s more important that you A.) Find a term that you feel captures the emotional and relational reality of your relationship, or B.) Find a term that you can use externally to refer to this person. It’s sort of one or the other, unfortunately; any term that carries enough meaning to feel right to you will very likely require a lot of explanation and definition. Any term that is conversationally simple is not going to feel complete to you. It is totally okay, when talking to your great-aunt or your coworkers, to use a simplified term just for the sake of not having to derail the conversation to explain the specifics of who this person is to you. It does not diminish your relationship, it’s fine.
As for specific words:
You can use “partner” or “life partner” - those words don’t belong to people who have a romantic/sexual connection (they have long been used to describe artistic collaborators, business partners, etc.) and you are free to re/define them.
You can combine existing words for terms like “Family Partner” or “Life Friend.”
You can just use each other’s names. I had one partner who ‘didn’t do labels’ so I just referred to him as “my Diogenes” and he referred to me as “my Zinnia.”
You can make up your own term by borrowing, adjusting, or combining existing roots. Lots of people have tried to tease out ‘different types of love,’ so that might be a good place to start. Check out this theory and set of terms. I knew some folks who just used the prefix “Co” to describe how they related to each other; it was meant to encompass words like collaborator, compassion, connect, etc.