A lot of good tips already in this thread; I'll add a few things that don't get mentioned as often:
Carousels are underrated for growth. They tend to get more saves and longer watch time than single images because people swipe through, and Instagram's algorithm weighs "time spent" heavily. A well-structured 6-8-slide carousel (problem → tips → CTA) often outperforms a single photo or even a Reel in terms of saves.
Instagram search is basically SEO now. Your captions, alt text, and even on-screen text in Reels get indexed for search. If people in your niche search things like "budget travel tips" or "home workout ideas," having those exact phrases naturally in your caption/alt text helps you show up there — separate from the algorithm's "for you" distribution entirely.
Collabs > cold outreach. Using the "Collab" post feature with even a small creator in your niche exposes you to their whole audience at once, and it counts as a joint post for both accounts (not just a shoutout). Way more efficient than DMing people asking for shares.
Don't rely on IG alone for discovery. A lot of steady growers are driving traffic in from outside, Pinterest, YouTube Shorts, or even a niche blog/SEO content, rather than hoping the IG algorithm surfaces them. External traffic that converts to a follow is often higher-intent than someone who just scrolled past a Reel.