I managed to miss that your system is a 600 gpd, not a 400 gpd. That little fact nullifies my recommendation. The pressure with a single
SW30-2540 would be 860 psi which is too high. I ran a series of calculations using Dow Chemical's ROSA program.
SW30-2540 860 psi 238 TDS
SW30-4040 560 psi 306 TDS
2 SW30-2540 in series 590 psi 426 TDS
These results were with inputs of 13% recovery and 25 GPH production.
The SW30-4040 study produced a low flow rate warning. The study with 2 SW30-2540 in series produced no warnings.
So, you have 2 choices, neither of which may be cheaper than the custom Sea Recovery membrane.