When to withdraw other faculty applications: conditional offer letter or contract?
Normally you should not withdraw your applications until you have received a binding job offer that you intend to sign.  If you are delaying signing it, that does not matter.
Depending on your circumstances, an offer conditional on a visa may be extremely risky.  Visa risk is unusually high right now owing to the pandemic, but even before the pandemic I have known people to wait for more than a year for a visa.