Ahh Construction PM here (well former)
Well, theres YouTube. Climbing channel? - Maybe a channel of your progress? - climbing gear review? With amazon links
Not very passive in the beginning but you will need to book out your weekends or evenings. To film.
You could start a climbing gear site? Low start up costs. Just the initial weekend adding products and setting up the website. Once you have manufacturers that you like. Spend some ££ on ads or pay an influencer to promote products on your site or use your YouTube channel?
Or even a personal finance channel on YouTube? These do well.
Create a course on on to climb or something that you're good at and sell it.