I believe I am somewhat in same situation. I have tried Nginx Proxy with Cloud Flare tunnel at the back and failed to get it working. I am not good with networking but some how it's working fine if I remove Nginx Proxy and let cloudflare tunnel call directly to Immich Server. It's probably due to my own ignorance but so far I haven't found any guide which can help me doing this successfully or help me resolve the issues I am having. I don't want to do any changes in my router settings.
Internet --> Cloudflare tunnel--> Nginx --> Immich