Hi [hiring manager],
I️ am touching base as I️ have received a job offer from another company, and after much consideration, have decided to accept it.
Please know this decision is not a reflection of my regard for [company] and your team. I️ truly appreciated getting to know you, and your team over the past weeks and still believe in your mission to [company's mission]. I️ look forward to watching you accomplish that mission, and if there is anything I️ can do to help, please do not hesitate to reach out!
Please let me know if you need anything else from me to close out this process. I️ am also happy to jump on a quick call if you'd like more insight into my decision making process.
Best,
[your name]