Whilst Asustor's ADM suits most people, it is possible to install ANY intel operating system on the nimbustor4/2 (as5202t/as5304t) as they run fairly generic intel hardware (asmedia sata controller, rtl8125 2.5gbe nic and ite system management chip)
in this gist I'll detail the steps needed to install and configure Debian11
Before proceeding you need to decide where you're going to install Debian.
Most people will choose to install it to a usb3 attached drive - either a usb3 ssd ( samsung t5 or similar ) or a usb3 hdd. This gives the most straightforward install ( just tell Debian to use the entire usb3 drive to install to ) and maximises available space on the NAS HDDs
The other option is to install Debian direct to the NAS hard drives - that means no extra drive hanging off the nas, and you can raid1 mirror the system partition across all drives for added resiliency ( though this is somewhat limited as the EFI boot partition is only on sda - so in the case of sda failing you'd need to boot off install media and rebuild the EFI partition). Downside is a more complex setup procedure and you'll need to go thru manual partitioning as opposed to guided (you'll also need a working knowledge of how to setup md raid1 in the partitioner)
NOTE - do NOT try and use a basic usb pendrive to install the system to - they have neither the write speed nor the write durability needed
Similarly do NOT try and install to the eMMC built into the unit - it's only 8gb ( which is too small for anything but the most minimal install ), and like pendrives, eMMC lacks write durability and write speed. Most importantly if you over-write the eMMC you will be unable to revert to ADM ( the eMMc has a minimal boot image on it which is used to recover the system - and there is no tool to restore it if wiped )
For this guide I'll detail the external system drive method
step 1 - change BIOS settings https://gist.github.com/johndavisnz/3b47df6a255b718b3e910e6331656cf6
step 2 - install Debian https://gist.github.com/johndavisnz/0d721c0b97615a5720660024f4a5f158
step 3 - install custom kmod for fan control and setup lm_sensors https://gist.github.com/johndavisnz/b5aae0236141666a77aac094701d7839
step 4 - install custom fan control script https://gist.github.com/johndavisnz/06a5e1aabaf878add0ad95669b3a0b3d
step 5 - install rtl8125 and i915 firmware, install OMV6 and Plex https://gist.github.com/johndavisnz/d4d8cf1304ebade08f924411d2c83327
if anyone's feeling adventurous I've just got this running on Rockstor on the AS5202t - the revised fan control script ( which gets hdd temps via smartctl ) works fine ( you just need to install sensors via zypper and and also install kernel-devel to build the kmod ).
Rockstor is very small and will actually run off the 7.3gb eMMc directly ( no need for a USB boot drive ) - but if doing that make sure you remove the 2gb swap partition it also creates on eMMc ( wastes space and could potentially cause a LOT of wear on the eMMc )