Is there a method to be able to load the firmware externally over SPI/UART and boot directly from the RAM (Flash-less solution) to save power, space and cost?
Yes, download the image from UART is supported. And RAM boot is also supported. We can refer to the WICED_SDK/Doc/WICED-HCI-Control-Protocol.pdf page 10-14 for reference.
BTW CYW20719 have 1MB flash inside, we can use the on chip flash to store the image.