Say no to extra work without sounding difficult

I'm being asked to take something on and I want to say no.

The ask: Run the new starter onboarding sessions every fortnight. It's landed on me because I did it once in July as a favour and it went well.
Who's asking: my manager's peer, runs another team, no formal authority over me but he's well liked
Why I can't: It's half a day a fortnight plus prep, I'm already behind on the migration, and honestly it's not my job and nobody's offering to take anything off me.
What I could offer instead: I could write up what I did in July so someone else can run it

Write me two versions of the reply. Both under 80 words, both as a message I could send today.

Version one: no, with the alternative I offered.
Version two: a straight no, no alternative, still not rude.

Rules:
- Apologise once at most. Not at the start and the end.
- Give one reason, the real one, in plain words. Don't stack three reasons, it reads as an excuse.
- No "unfortunately", no "I'd love to but", no explaining my whole workload.
- Don't leave a gap they can negotiate into unless I asked for one.

Then tell me: whether my real reason is safe to say to this person as written, or whether it needs softening, and which specific words would cause a problem.

What you get: Two short replies you can send, and a check on whether your real reason is safe to say out loud.