If it was me I’d get rid of the Sargent and start again. All the faff of trying to disable the dc-dc charger and the 240v charger etc would just drive me daft.
Surely it’s easier to get a 12v fuse panel and a pre made switch panel and rewire your current 12v stuff to it.
240v hookup cable and consumer unit etc only
costs about £60 to put in.
I see Fogstar are now doing a 230ah seat base lifepo4 battery for £800.
Seat Base Lithium Leisure Battery 230ah | Grade A EVE LiFePO4 cells, JBD BMS, Bluetooth & Heating. Just £799.99 with a 10 year warranty.
www.fogstar.co.uk
Get a decent dc-dc charger, but as your wiring is already geared towards a seperate MPPT controller I wouldn’t bother paying the extra for a dc-dc with that built in. That should get you up and running and you could still use your existing solar panel for now with a new MPPT controller.
Upgrade solar at a later date and install AC battery charger at a later date (but run the EHU cable when you have it stripped out for the dc-dc install).