Don’t treat all 500 the same; sort them. Google itself says that if you can’t write a description for every page, you should prioritize, and at minimum cover your most important and most-visited URLs. That is the rule in miniature: hand-write the descriptions for the pages where the click is worth fighting for, and let Google generate the rest.
The pages that earn your time are the ones with commercial intent and real traffic, your money pages, your high-converting pages, and anything pulling meaningful impressions, where a sharp, query-matched line can win clicks a generic auto-snippet would leak. The long tail is where hand-writing is wasted effort: low-traffic posts, utility pages like privacy policies, and old content that will never rank in its current state. Google rewrites most descriptions anyway, often more than half, sometimes showing several different snippets for one page in a single month, and on those pages it will happily pull a query-relevant line on its own. Writing a careful description for a page nobody searches for, and that Google will overwrite regardless, changes nothing.
Two cautions while you sort. Don’t paste one duplicated description across the low-value pages to feel thorough; that is worse than leaving them blank, because Google won’t show identical snippets and will just generate its own. And do write a description for any page that is mostly a list of links or embedded media, since Google has little text there to build a snippet from, and a written line gives it something to use.
Split the 500 into two piles. One is hand-write: commercial, high-traffic, and critical pages, where the snippet is worth controlling. The other is auto-generate: everything else, left to Google. Spend your hours only on the first pile, and stop maintaining descriptions on pages that can neither rank nor hold the snippet you gave them.