Hi,
that is an interesting setup and UseCase.
Yes, I am also gathering the Power flow at grid level with an MPM3PM meter (openWB EVU-Kit).
The reason I decided not to put everything in node-red is at the end risk based.
In order for EV charging to be based on available access power at grid level, openWB needs the data in any case.
However I am uncertain how many simultaneous connections the modbus devices would allow (and at what reading frequency - ie. SMA is very conservative in their docs)
So using both (node-red or openWB) simultaneously over modbus has a risk of overlaoding the inverter/grid-meter modbus readouts.
I have a test flow, based on integration via mqtt from openWB and with modbus-tcp for connecting my SMA inverter and grid meter directly and show these in parallel on the dashboard.
The results are showing that openWB is doing a good job, although only reading out the data in 10sec intervals (I can share it, if you want).
If using modbus with node-red alone, then feeding this back to openWB via mqtt is certainly possible, but I decided to not do that until I need better resolution, over the 10sec interval openWB provided.
For integration with node-red, I would actually prefer using mqtt over anything else. This is the safest option, IMHO.
Therefore another integration option with node-red for you could be mbmd from volkszaehler.og.
Its a modbus deamon that can read many meter and inverter types, then pushing to mqtt (for influxdb, also telegraf has modbus plugin now). Also openhab and iobroker, FHEM have many integration option for meters and offer mqtt-support.
As for your SMA inverter running bluetooth only, I have no clue. Maybe upgrade with the webconnect module?
The challenge with solcast in your case, I see is pv-tuning.
You will need the PV measurements from your setup in order to do that and get a good result.
Edit: BTW, this is how my dashbord looks like while charging from PV:
