You might just have an issue with Cassia cinnamon. It's more commonly available, but it's dangerous even in small quantities. You might try Ceylon cinnamon instead.
https://www.healthline.com/nutrition/ceylon-vs-cassia-cinnamon
Exceeding the upper limit for coumarin is easily possible if you are consuming a lot of cassia cinnamon. In many cases, just 1-2 teaspoons could bring someone over the daily limit.
In rodents, coumarin is known to cause kidney, liver and lung damage. It may even cause cancer. In humans, there are isolated incidents of similar effects (11Trusted Source, 12Trusted Source).
You probably use a lot more than that in homemade Horchata, but also that's just the recommended limit. OP might be more sensitive than most and can only tolerate much smaller quantities. They also specifically said they can't have it in large quantities