AFAIK the boot order is set in SOC’s BootROM (SPI -> eMMC -> SD -> USB) and cannot be changed.
I did some hacks in u-boot for Rock Pi 4 in Armbian to always try to boot from SD if it is present but first u-boot stage is still run from eMMC.
If you can boot as far as u-boot proper from you eMMC you could stop autobooting - hit any key while in u-boot - somewhere around the Model: ... line.
At the prompt you can then examine mmc/sd devices:
=> mmc list
dwmmc@fe320000: 1 (SD)
sdhci@fe330000: 0
=> mmc dev 1
switch to partitions #0, OK
mmc1 is current device
=> mmc info
Device: dwmmc@fe320000
Manufacturer ID: 3
OEM: 5344
Name: SC32G
Bus Speed: 50000000
Mode: SD High Speed (50MHz)
Rd Block Len: 512
SD version 3.0
High Capacity: Yes
Capacity: 29.7 GiB
Bus Width: 4-bit
Erase Group Size: 512 Bytes
… and boot from SD:
=> setenv devnum 1
=> run mmc_boot
switch to partitions #0, OK
mmc1 is current device
Scanning mmc 1:1...
Found U-Boot script /boot/boot.scr
This works for sure in Armbian but should work as well in Radxa’s images.