I have a holsteiner mare that I recently switched to barefoot. Although i don’t compete in eventing she used to with the front two hooves shod, her front hooves were shod her whole life and it certainly did not make her feet any weaker. Her hooves are very strong and when barefoot trimmed every 4-5 weeks. What I would suggest for you is to find a barefoot farrier and get his/her opinion on wether your horse could go barefoot. The other thing would be that since your horse has brittle hooves I would put her on a hoof supplement to make trims last the max amount of time. Also, what level are you competing at? I think a horse could easily compete up to novice without shoes, but once getting into training id definitely want the shoes for the extra grip. You could also try pulling her shoes this winter (when you don’t have as many competitions) to see how she responds and to build up the strength in her hooves before show season hits next year. Hope I have you some useful advice! Good luck eventing with your gorgeous mare!