Tutorial guide how to set a Cloudflare R2 S3

what is exactly you doubt ? a full tutorial would require a r2 subscription wich i don't have.

in simple steps:

1. go to the cloudflare dashboard
2. create a bucket
3. create an token to authenticate on the s3 api: https://developers.cloudflare.com/r2/api/s3/tokens/
4. go to siyuan configure the s3 sync with the information gathered on previous steps
5. after configuring the conection information go to cloud sync dir and add a directory for the sincronization

in general i think that under normal use "not adding/modifying more than 20k assets/documents per day" the billing probably would stay on the free tier.
for comparison i have a minio instance, my bucket has 615MB with a total count of 23,305 objects,
on R2 this would stay on the free tier
if the chages are much frequent i would recommend increase the sync time from the default 30 second to avoid surpassing the 1mm write operations on the free tier.

Helped me. My fear is putting my Paypal account there without limits and due to an error in the sync they will charge me a high amount because there is no way to set a limit there.