Here we have another case of subject-verb disagreement in the original text (“all things ... which was expedient”). For the 1837 edition, Joseph Smith edited the singular was to the plural were. The critical text will follow the earliest reading, despite its ungrammaticality in standard English. For a complete discussion, see subject-verb agreement in volume 3.
In this particular instance, we have a plural antecedent (“all things”) for the relative pronoun which. The original text consistently used the singular in the relative clause “which is/was expedient”, no matter whether the antecedent was singular or plural. Besides the example here in 1 Nephi 17:30, we have the following:
The singular form of the be verb (either is or was) has been edited to the plural are or were in all these cases where the antecedent is plural.
Summary: Restore the singular verb form in the relative clause “which is/was expedient”; for this construction, the original text consistently uses the singular form of the be verb no matter whether the antecedent is singular or plural.