Then, together, do some searching for perspectives on why being everything to one person isn't feasible nor healthy in relationships

But these perspectives aren't synonymous with opening up a relationship. Very few people are "everything" to a partner. Healthy partnered people have family, friends, neighborus, aquaintances, coworkers, etc - a long list of people who can "be" things for them.

There's nothing in OP's post that suggests they are out of people to "be" something for. But if this was the case it would be smarter to start with increasing their support network before they go non-monogamous.

Only a wish on both of their parts part to open up their relationship would be reason to sit down and search for reasons to do it. If one partner doesn't want it, it would be similar to creating a PUD situation.