Sadly once you are on a spammers hit list there is not much you can do.
Like you I also have an email address that was ONLY used for my family and a few close friends (so only a dozen or so people). I never used it on web sites etc. as I have other email addresses I use for web sites, banking etc.
About a year ago, out of the blue, it began to get spam and I now get about 50 spam a day (which all goes in to my spam folder).
All seems to be US based (Trump, US Medical Care, US car repair etc).
I keep hoping that one day they will catch the b*****s who are sending it and it will stop, but it has been going on for over a year now.
I think my only choice is to set up another email address and tell all my relatives and friends about my new email address.
I have no idea how they got my email address (and it is not JUST based on my name like John Smith or whatever it is more obscure than that).