I was intending to use the taxonomy to create hierarchical pages, which I initially got around by just setting the Menu Position value for the page as such:
page.As<MenuPart>().MenuPosition = _navigationManager.GetNextPosition(menu);
//appended ".1" to menu position for subpage, etc.
Someone I work with answered my original question though:
//create local var corresponding to desired preexisting taxonomy
taxName = _taxonomyService.GetTaxonomies().FirstOrDefault(t => t.Name == "Taxonomy Name")
//updates Field Name in page object to IEnum<TermPart> returned by second argument
_taxonomyService.UpdateTerms(page, taxName.Terms.Where(term => term.Name == "Term Name"), "Field Name");
Hope that helps anyone who might have the same issue!