If the 5A is extended with an SPI-flash module on the bottom side, one can flash a generic/modern/new bootloader into it, so like GitHub - edk2-porting/edk2-rk3588: EDK2 UEFI firmware for Rockchip RK3588 platforms · GitHub and then it is like a PC; can boot from CD-ROM .iso image on USB-stick etc. It must be very new variant of the distro (rolling release, kernel >= 6.19). So Debian Sid or openSUSE Tumbleweed for example. I use Debian Sid kernel and that UEFIv1.1 with both a RK3588 and RK3588S SBC’s and works nice. Note is that certain HW like video encoders are not working, but you can browse the web and plenty of server tasks etc.
If no SPI-flash, it gets tricky as re-partitioning of SD-card might be needed for the space at the beginning of the SD-card (first 16MiB) where the bootloader must be located. But can work as well.
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Should it work out of the box with basic media and armbian‑imager, or do I need to look for some custom SPI first? 
There isn’t much information about this on the official download pages or in the imager, and the images I’ve burned simply don’t boot. I’m sure the right combination exists, but when Armbian aims to replace or mimic the Raspberry Pi approach, users don’t expect a standard image for a supported board to fail at first boot.
Even a small note with a link to the correct instructions — or mentioning that an additional SPI‑update image needs to be flashed first — would already help a lot. I’m not blaming anyone; as stated, it comes with no support or warranty. It’s just not yet as seamless as the Raspberry Pi Imager experience.
In your handling SBC’s, you need to separate bootloader/fimware and OS(Linux). Problem with images is that they are just 1 thing. SBC’s for which it is unknown whether they come with factory-flashed bootloader or not, are a problem in that sense. So that means the majority as most cheaper ones simply don’t have any storage chip.
From Radxa’s O-series and Q-series you know they always come with UEFI bootloader. Same for RPi’s, but it is proprietary bootloader. All others, the question is simply: What have you bought?
Does your 5A have SPI-flash or not?
If yes, and you put EDK2 UEFIv1.1 in there, you can use standard UEFI based images. Note that there aren’t many of those as the assumption is that you use a normal installer program like you install Linux on an x86 PC.
If no, you need some place where the bootloader can be stored, usually SD-card or eMMC and it needs quite some space (usually 16MiB is reserved). And that is the problem with generic UEFI based images, the first partition starts already at 1MiB (sector 2048). That is why I mention re-partition.
This is clear, I’m sure that with right bootloader it should work. All I wanted to say is that all debian 13 require something more than just burning sd card image when ubuntu will just boot. Any tip for that would be helpful on downloads page or in imager. I just expected to have everything that is needed inside image on standard boot medium.
BTW: 5A comes with SPI as eMMC module.