This isn't a sign that you should have written a manual years ago and failed. It's what happens when a business is built by one person who was busy doing the work. The knowledge accumulates because it has to, and it stays put because writing it down never feels like the most urgent job.
Write down the few things only you know
Start with five. Not the whole history of the business, just the five things that, if you were unreachable tomorrow, would cause the most trouble: how a certain process is done, where the important documents live, who you call when a particular thing breaks, what the usual price or the usual terms are, how the inbox or the diary is meant to be handled. Write them in plain language, as if you were telling a competent person who doesn't yet know your shortcuts.
Put that note in a place someone else can actually open, in an account the business owns, not only on your personal phone. If there are logins, use a proper password manager in the business's name, not a list in a drawer and not your memory.
Photograph or scan the important papers and give the files names a tired person could search for. "How we do X" is a better title than "scan0032". If you already have a pile of old documents, you don't have to sort them all this week. You do have to put them in one place, and start giving new things better names.
When you next do one of those five things, check the note and mend it if it's already drifted. A short living page is more use than a long handbook you never open.
If other people work with you, ask them what they always have to come to you for. Their list will not be the same as yours, and that's useful.
A simple place for the basics, even when you're out
We'd write up how you do things and make your old documents searchable. The writing would be done with you, in your words, so it sounds like the way the place actually runs, not like a procedure from somewhere else. You'd approve what's written, and you'd decide who can see it.
This would live in accounts you own, in tools you already pay for or in a simple shared place in your own tenant, on your card, in your name. We'd only have access for as long as you want us to. We charge for the writing and the filing, and for keeping it current if you want that to be part of monthly care. If we step back, the notes and the documents stay where they are. The place can still run, because the basics are no longer only in one person's head.
You will still be the person who knows it best. That doesn't change. What changes is that a day you're out is no longer a day everything has to wait, and that the knowledge you've built up has a home that isn't only you.