Cross-promote / recommend with other newsletters.
Set up a referral program.
Repurpose and share your content everywhere.
Send it to all your family and friends and ask them to share.
Guest write for bigger publications.
Do a giveaway on social media.
Answer questions online (quora, reddit, twitter, etc) and link to your newsletter.
You can even still do paid ads if you set up Sparkloop's Upscribe or Beehiiv's Boosts.