Right now, Linux 6.6 is fine. A bit old but fully supports all relevant software. We currently ship RK3588 SoC images with a Linux 6.1 vendor kernel, so … However, for RK3588 mainline Linux support is there and becomes better, so at some point we can move to latest mainline Linux or latest LTS. If the XpressReal T3 gets no vendor kernel upgrades and no mainline support, in 2 years or so, Linux 6.6 will likely start to get issues with recent software that makes use of particular new kernel features.
So right now, you can use it perfectly fine with e.g. the Armbian image, optionally tuning it into DietPi. But based on the rough look at the sources, I guess chances are high that its kernel stays at 6.6, which is why I wouldn’t recommend to buy it. We have seen similar in some other cases, Sparky SBC (stuck at Linux 3.16), NanoPi 2 series (stuck at 4.4) and NanoPi 3 series (stuck at 4.14, actually 4.4 but Armbian invested efforts to rebase it onto 4.14 at least). All those cannot even run the oldstable Debian 12 Bookworm properly, which is why we need to drop support for them next year when we drop Debian 11 Bullseye support. And container engines like Docker + other such software which makes use of recent kernel features (including modern filters/firewalls) stopped working long before. And this is so sad, since especially the NanoPi 3 SBCs are pretty fast with their octa-core SoC. But if vendors contribute nothing to mainline support, making it extra hard for volunteer developers or interest is too low, I’d even vote for explicitly not buying such hardware for the possible market power effect 
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Because their builds do not support recent libraries or something like that? The kernel is the same for SBCs on all Debian versions, so when compiling it yourself on Trixie it should work.
From which boot media did you try it? SD card, eMMC, M.2 NVMe SSD, USB? I have one here and will test. Since you mention the Bookworm image only, you did not test the Trixie one, did you? No need to do, same kernel same bootloader etc, hence I would be surprised instead if the Trixie image from same boot media boots, but the Bookworm one does not.