Hey @Chris_Hill welcome to the forum.
Although Octopus have been guilty of oversizing Daikins in a few properties, I do commend them for listening to customers and replacing the units when presented with empirical evidence of such oversizing. I’m not sure there’s many other large or small installers who would take that hit.
Hopefully they are learning their lessons about the importance of accurate heat loss surveys and how engaged some home owners are going to be with regards to how their system is performing. I do worry that all the data they are getting out of the Cosy heatpumps will stay proprietary and won’t be shared with the user, time will tell on that I guess.
You absolutely can feed ESPAltherma data into EmonCMS, you just need to get the MQTT feeds in to the system. Or if you have an emoncms.org account a member of this forum (@Phil_E) added functionality for ESPAltherma to be able to publish directly over HTTPS: Daikin Heat Pump ESPAltherma / HomeAssistant installer bundle for emoncms - #59 by Phil_E