One thing worth knowing before you write it: clients you leave well are a genuine source of future work.
I have been rehired twice by people I resigned from: once for a project that suited me much better, once as a referral to someone else. Neither would have happened if I had disappeared or priced myself out.
What makes the difference, in my experience:
Never leave mid-thing. Finish the piece you are on, or hand it over so completely that finishing is straightforward for whoever is next.
Write down what only you know. The undocumented decisions, the workaround for that one thing, where the credentials live. A page or two, and it is the difference between a clean exit and three months of one-off questions you feel obliged to answer for free.
Be genuinely helpful about the replacement. Recommend somebody if you can. It costs you nothing and it is the single thing people remember.
Do not use the exit to deliver feedback they did not ask for. The urge to explain the weekend messages is strong and it converts a clean ending into an argument. If they ask why, be brief and factual and stop there.
And on the guilt about leaving decent-but-disorganised people: it is a business relationship ending with notice, not an abandonment. They will be fine.