Does anyone have experience with multiple inboxes (50+) and Smartlead?

Yes I have 8 years of experience

Is warning them up for 14 days strictly necessary?

Ya I do, new mailboxes warming up is necessary, in order to avoid reach spam box. Also, it's good to warm the mailboxes every 2 weeks or you can start warm up when you see domain reputations goes down.

How is warming up different than just throttling my emails sends for 2 weeks? Like sending only a few per day?

Warming up = slow, progressive increase + targeted, high-engagement recipients (builds trust). Throttling = just limiting volume (doesn’t guarantee reputation building).

But Smartlead warm up emails also get bounced. Try with low volume for warm up