So, I got my Delta MAX back from their support after it was stuck and decided to share my feedback on this.
The RMA was a bit painful but in the end all went well.
I’ve got my unit back plug it in, saw there was an update, click on the button, went away and came back, the unit seemed stuck again.
So I went back on the app, pushed again on the update button, it continued where it was.
When I came back, it was finish but still on 1.2.6.19 instead of 1.2.6.29.
Now, it says it’s up-to-date 
Anyway, I raised a ticket to explain the situation to them… and to ask one more time about other API access ;-).
While I was there, I made them the following suggestion:
"What could be great is to get a bypass mode (only provides the power requested by the devices plugged on the battery) and an off-grid mode (use energy only in the battery even when plugged in the wall).
With these 2 features available through the App or the API, users could use automation to charge the battery only with excess solar from their rooftop !
- When PV production > house consumption = normal mode: battery charging if needed + power for devices on it
- When PV production < house consumption = off-grid mode, until it reaches 20%
- When PV production < house consumption and battery < 20% = bypass mode, until there is excess solar again.
It could greatly help people auto-consume their energy !"
Not sure if anything will come out of it, but at least I’ve tried :-).