Unfortunately, I assume that this product has been canceled a few months ago, when Intel sold their NUC business to ASUS.
There are a few small computers that support ECC and which use obsolete Intel Tiger Lake or Tremont-core-based Intel Elkhart Lake CPUs, but those CPUs are a dead end, being slow and supporting instruction sets that are different from the current mainline Intel CPUs, so I would not recommend any of them.
The best remaining choice depends on which is more important, the size and the power consumption or the price of the server.
For very small size and low power consumption I am not aware of any good solution at a reasonable price, because even when some of the Arm CPU SoCs or Intel or AMD mobile CPUs support ECC, I have not seen any such computer board that includes the ECC support. There are some industrial computers with ECC, but those are expensive for what they offer.
If only the cost is the problem, and second-hand servers are avoided because the server to be bought is intended to be used for many years, then a server with desktop Intel or AMD CPUs must be used. The MBs with the Intel W480 chipset are expensive, so the cheapest solution is to use one of the AM5 MBs that specify ECC memory support, e.g. from ASUS or ASRock Rack, together with one of the cheaper Ryzen 7000.
The recompilation of the Linux kernel may be more difficult, because the right configuration file and modules must be selected before doing it, but it should be possible as most support for RK3588 is included in the mainstream kernel. Also U-Boot (the boot loader that loads the Linux kernel) should be recompilable from sources.
The hardware is more trustworthy than that of Intel or AMD computers, because it comes with the complete schematics and the technical reference for RK3588 is much more complete than for any Intel or AMD CPU.
A hardware backdoor could have been implemented only in the Ethernet interface of RK3588, but that is not used in NanoPC-T6, which uses Realtek Ethernet interfaces on PCIe lanes. Any hardware backdoor in those would have required a close and secret cooperation between a major Taiwanese company and a major mainland Chinese company, which is unbelievable.
This is normally provided in all Arm SoCs by binary blobs. Nevertheless, at least for the Mali GPU there is a reverse-engineered driver in the Linux kernel, which might be usable with RK3588.
In any case, this hardware acceleration is the same in all RK3588 boards, regardless of the vendor, it is not specific to NanoPC-T6.
Software from board manufacturers shouldn't be used anyway, if not because in a few years it is often discontinued and not updated anymore because they're pushing newer models. Thankfully we have Armbian and Dietpi which are the distros of choice for all boards that don't run major PC oriented distros (and a nice alternative for those that do). The number of boards supported by these two distros is astonishing:
https://www.armbian.com/download/
The NanoPC-T6 is already supported by Armbian build system:
In this class, there are many models with RK3588, having e.g. dual 2.5 Gb/s Ethernet ports (with the possibility of attaching more Ethernet NICs on USB 3 or on PCIe M.2 adapters) and supporting PCIe 3 x4 M.2 SSDs and/or eMMC (and the attachment of more SSDs on USB).
A cheaper option, but with much slower peripheral interfaces, is the new Raspberry Pi 5 model.
Nevertheless, a homelab server with an ARM CPU makes sense only for developing Aarch64 applications.
For just doing the job there are many small and cheap fanless computers with Intel N100 (4 E-cores) and 4 to 8 2.5 Gb/s Ethernet ports (typically sold on Amazon as firewall appliances). For only a few dollars extra it is possible to find much faster cheap small computers (from companies like Minisforum or Beelink) with older AMD Zen 3 CPUs, like the 6-core Ryzen 5 5600H.