Its quite easy to fix the Ecoflow charging, you need to run a hard line from the battery to the rear, but also add a step up DC to DC charger..
So here is food for thought.
The Delta has 1200wh of storage this is effectively 100ah of Lithium or 200ah of Lead Acid. The max input it will draw is 400w from the solar port. If you were to install a 12 to 24 or 12 to 48 400w DC to DC supply with a direct feed to the battery, then you could charge the unit fully from empty in 3.5 hours.
For example -
Victron Orion 12/24 20A 12V to 24V DC-DC Converter Non Isolated - Low Energy Supermarket
The problem with all the power stations is they need more than 12v's to charge effectively from there PV port.
All you really need is to sort the supply side.