I get maybe 3 or 4 spam calls a year. I've had the same number for over 20 years and I've given it to thousands of people. Like you, I only give it to real actual people who I want to call me, though many of them were strangers. I never placed any restrictions on how they store it or made any other effort to keep it private. Since 2009 or so I have had a google voice number in a different area code that I use to fill out forms and give to strangers, but since blocking calls to it from that same area code I let it ring through to my main number now and it also gets virtually no spam (it's possible the 3 or 4 I get are actually from this number).
My main actual trick has been to be discerning in the things I sign up for and never filling in information that's not required.
The most egregious sources of spam I've found are the government. Get your name anywhere near a property deed and you'll get a barrage of spam from companies that think you're new to the neighborhood.
Think you might want to save on paper and give out your email address when you register to vote? Dear god. San Francisco doesn't give voters the ability to only receive correspondence from the Department of Elections. The spam is insane. Obviously you get all sorts crackpots running for office that'll spam you endlessly. For a while I was even getting email about pet clothing from some guy. Turns out he ran a failed mayoral campaign and then turned around and used the voter rolls to promote his business (because clearly I want a tutu for my non-existent dog).
And, sure, you can remove your email address from the voter rolls but not your physical address (so that spam will continue unmitigated). San Francisco is an extra level of special here as it doesn't even bother with HTTPS for their Department of Elections stuff.
My main actual trick has been to be discerning in the things I sign up for and never filling in information that's not required.