How can security leaders in smaller organizations stay informed about emerging threats if they don’t have access to formal threat intelligence feeds?

As others have mentioned - reports from CISA (or your local equivalent) are free and useful. Join an industry forum so that you can share information and observations.

Have a Threat Profile prepared for your organisation and updated periodically.

I also want to point out, don't lose sight of the fundamentals. Inventories, Patching (particularly the edge) and hardening, Monitoring, ability to Respond & Recover. The majority of cyber threats are still hindered by getting the basics covered.

Most strategic threat intelligence relevant to planning is published for free by most threat intelligence shops, you don't need feeds for this. You might want to ask someone to compile you relevant things once a quartner/annually.

One way is to subscribe to some of newsfeed such as:
https://www.infosecurity-magazine.com/
https://www.bleepingcomputer.com/
https://www.cisa.gov/news-events/cybersecurity-advisories

Microsoft has good blog site as well at https://www.microsoft.com/en-us/security/blog/ which has a section on Threat Intelligence and Security Insider.

Attend Black Hat conference if possible. Hope this helps.

There is an easy way to answer this question: Subscribe to CISA's threat advisory service (for free). Their threat analysis is world class and yet, remarkably easy to interpret. Link here: https://www.cisa.gov/news-events/cybersecurity-advisories?f%5B0%5D=advisory_type%3A94

When you run a startup, creativity is essential. AI is great for that. With the right prompt, it’s amazing how it can process information and deliver exactly what you need.