Not opting into marketing spam (or opting out immediately in the website's settings) takes care of 99% of it for me. On a set of ~250 accounts I will maybe receive one marketing e-mail per month (and they quickly get a complaint to their data protection officer to discourage that happening again).

For transactional stuff a set of rules to automatically read+archive stuff that you need to keep (invoices, receipts, order notifications, etc) but don't necessary need to action.

This leaves you with only the relevant stuff in the inbox and makes it much easier to do "inbox zero". I will typically get less than one email a day actually landing in my inbox using this technique.