Actually there is limited returns but a nvme on a old core2 duo or even celeron its not great as the CPU bottlenecks the nvme more than other storage bottlenecks the CPU.
If you have a server style SBC than fast storage can be served, application SBC need a cpu matched to storage.
The rk3588 isn’t that fast a application processor and like others say and eMMC 5.1 can provide up 400 MB/s eMMC isn’t slow just the variants and interface used often are due to cost.
Rockchip have given the rk3588 eMMC support and not Universal Flash memory UFS support most likely because they think it is out of balance with the CPU and cost.
Its balance and a RK3588 is likely to be the 1st real low end desktop sbc experience once we have full MESA drivers and various hardware have been pushed upstream.
We have a release approx Nov for early adopters but likely it could be another year or 2 down stream.