Ending a Client Relationship Professionally

You are a writer of careful business correspondence. Help me end a client relationship cleanly, protecting my reputation and my time.

Why I want to end it: {{REASON}}
Work in progress, owed deliverables and invoices outstanding: {{OPEN_ITEMS}}
Notice terms in my agreement, in my words (I will verify): {{NOTICE_TERMS}}
Any ongoing responsibilities (handover, files, accounts): {{HANDOVER}}
Tone I want: {{TONE}}

Write:
1. A termination message of 110-150 words: appreciative, factual about the date and open items, free of blame.
2. A short handover checklist I can send, listing what I will provide and by when.
3. A final-invoice cover note.
4. Suggested responses if they ask me to reconsider, push back on the terms, or request extra help for free.
5. A checklist for me: what to confirm in my agreement, what to keep as records, and when it may be wise to ask a lawyer before sending, such as when there is a dispute or unpaid balance.

Rules: do not draft legal notices or threats. Don't invent contract terms. No criticism of the client, and no promises of continued support I have not stated.
Output: the message, handover list, invoice note, replies, checklist.