I can respect and agree with the goals but, in my personal opinion you’re going in the wrong direction.
First and foremost, compare ARM vs x86 is a matter of OSes: in the future ARM will completely wipeout the x86 architecture because is more modern, flexible, tweakable in the design and can be re-designed from the ground up anytime needed due to the RISC roots. I don’t know if really RISC-V can become soon product, “freeing” electronics from royalties and patents but… it is a even longer road than ARM.
However, the x86 corral is still not there: unless there’s at least some project behind, ARM won’t arrive on general purpose hardware for become a product; of course there’s this wonderful unicorn called Raspberry Pi, but still an extremeli niche product.
Next, your assumption “If the base of NS8 running on Raspi4 it should run on a T630 or T530” is formally correct but in reality not. Can, but should not, in my opinion.
Containers request the power on top of the operating system, and NS8 acts like a container orchestrator, allowing all asserved computers to run containers for the integrity of the system. It’s pretty interesting concept if you can have more than one hardware and you want to start a service, end the configuration, test it, then move it from the lab hardware to the production hardware. Also, containerization allows to have services and applications just like dongles: OS provide computational power and connectivity but as software they won’t share that much, allowing a lesser degree of isolation than a VM, but an higher one than a service. Sorry for the “lecture”, maybe you already know all of that.
However, to run the underlying OS and the containers computational power is needed; a bit more, from two to three times at least than the one of that thin client in my opinion.
Which could run beautifully CentOS7 and NethServer on top and i know because a Nethesis provided PCEngines.ch APU2 units as Firewall devices; 6W against 35W of TDP, but the efficiency is still nice.
Again: keep your hardware, but try NS7. If you feel the urge for NS8 IMHO you need more horsepower under the hood, and the suggestion i made about CPUs were related to performance goal, not explicitly to that hardware.
If you’re into thin clients, t740 can be a more interesting option.
With a Ryzen-based embedded CPU, for less than 30% more power delivers close to three times the computational power, double the threads, supports DDR4 (so more performance for less consumption). As T730 allows to add a PCIe card, and if you don’t need extra network connection, maybe a PCIe to NVMe adapter could give you more room for adding storage (the provided eMMC module is quite useless) but consider to check before take the faith leap of “it will work”.
With a t740 in my opinion the testing of NS8 could be a viable task, even a home/soho installation if storage is not the primary goal.
Sorry to nag you with all my assumption and ideas, i hope that time reading at least was not considered wasted.