I'm single and living alone, no family in the picture. I have a very small circle of friends I love and who I trust implicitly. With their permission I ask them if it's okay to share some important things with them, in case the worst happens. Spare keys, contacts, etc. etc.
Those friendships might outlive any romantic partnership I do have in future. So I don't think it has to be a partner specifically. Of course, there are still some things that you might not share as openly even with that trust in place, but that doesn't stop you from having a backup plan.
Couldn't you have that binder laying around in your home anyways? I imagine my family would be able to gain access to my home if I die (even though not one of them has a key).
I downvoted this at first, but I've undone that and am going to respond.
If you have family, extremely close friends (as adults, life-long friends), these can be options. Consider keeping your 'binder' in a safe deposit box and setting up access via your bank.
If not, an attorney or even CPA may be able to keep this information for you.
An alternative is to have your attorney be responsible for executing your will, but keep everything else in the safe deposit box and put the instructions on how to access in the will. That way your attorney can access only if you die and they assume legal authority for exercising your will.
If you don't have anyone you will leave your assets to, the attorney will be the one liquidating your estate as per your will.
Collate this same information, encrypt it and then use [a key sharing algorithm](https://en.m.wikipedia.org/wiki/Shamir%27s_Secret_Sharing) to split this encryption key across a set of semi-trusted contacts.