Do you often sign up for new services using your actual email? In gmail, I get spam in my primary mailbox maybe once every few months, usually after I've signed up for something. After marking mails as spam a few times, they are usually sent to the spambox automatically and I have a tidy primary inbox again for a long time.

I’m sure the source is LinkedIn ultimately. They reference my job and things from there and pretend to have carefully considered my posts (just LLM generated BS)

Especially if you signed up for something, rather than just unsubscribing marking it as spam is a dickish move given that it has the potential of that mail also being filtered for people wanting to receive it.