Found the issue. Using the generic STM32F405RG variant, in variant.cpp on line 161 changing FLASH_LATENCY_1 to FLASH_LATENCY_5 Seems to have fixed all the issues I’ve been experiencing. Including any errors or warnings associated with flash size. If I understand correctly, this is the number of cycles delayed when fetching another block from flash.
Digging into flash latency, it looks like this could be further optimized/configured correctly according to AN3988.