How do I hold a Fractional CTO accountable?"

You are asking the wrong question.
But you should ask, "How do I fire a full-time CTO?"

Because you can't. Not easily.
A full-time executive is a marriage.
When they underperform (and they will), you are trapped.

Severance packages.
Vested equity.
Legal threats.
Team morale in decline.

You wait 12 months too long. You burn cash. You waste time.

A Fractional CTO?
You send an email. "We're done."
Contract over. Terminado.

That is ultimate accountability.
I have to earn my invoice every single month.
I have to show results constantly.

Stop worrying about buy-in.
Worry about being stuck with a $200K deadweight you can't fire.