question

Not familiar with Harold Halewijn's but I'd just make sure you've got a fast acting fuse between those parallel lifepo4s just incase something funny happens.

(like if BMS 1 went off for some reason, then battery 2 and 3 are fully charge then BMS 1 turns suddenly turns back on and there's inrush current to battery 1, you'll want a fuse there just incase). Given flat curve of lifepo4 you should be fine especially with only 3 packs in parallel but better safe than sorry.

As Kev mentioned below Node red is exactly what you're after in terms of discharging to grid etc. However you can also go the route of managing that externally via another system.

(I personally don't like using an external system to manage this. As then you're reliant on another system, ie say you turn on "exporting" via the domoticz but then the domoticz system crashes so never turns off exporting. I personally prefer to have automation setup on GX devices using node red running on venus os large itself and then if I want to pass in external info or changes from another automation system I do it via node red just so if that external system does go down for some reason, then I've still got Node red to turn something off in the event the "off" message is never received, if that makes sense)

However if you do want to go the external system route anyway, you're spoilt for choice in terms of receiving the data. There's MQTT, modbus TCP on top of node red. You can even make raw python scripts (or any other language) to read data from dbus directly and then pass to external systems however you prefer if MQTT/Modbustcp aren't what you're after).