I have connected 435W panels to each of the two MPPT connections. Works great. The panels very rarely have more than 400 W, but this is not due to the EcoFlow, but more to the installation angle of the panels, the weather and the outside temperature... You need the right cables from EcoFlow so that the charging power is not limited by the plug, which is a protective mechanism for charging from the car alternator/battery.

Not for the Delta 2. With 2 × 400 W you either exceed voltage limits (if in series) or waste unused capacity (if in parallel). You’ll still only get 500 W solar input maximum.

Hello, I have combined the Delta 2 with the balcony power station from Ecoflow (2x400W panels). The inverter feeds a maximum of 800W to the battery and 600W to the grid.
More watts per panel tends to be better, as charging is faster. Ultimately, the space available for installing the panels may determine how many watts they can have.

The Powerstation can handle 1000 W solar input

2x 500W

As high as possible, you could even theoretically use the 2x 520W from EcoFlow.