I do wonder if you could solder 2 more memory chips on one of these cards to make it 8gb
Only if you swap the ASIC to TU106. TU116 won't run 8GB with that configuration unless it were specifically setup to run higher density modules at 128-bit instead of 192-bit. You could theoretically increase it to 12GB by replacing the current modules, though.
The reason for those empty spots in the first place is because the PG160 board layout is also used for the RTX 2060 and RTX 2070.