Axzez Interceptor carrier board for RPi CM4 gets 8-port PoE+ board
cnx-software.com
cnx-software.com
I was also looking forward to the Radxa Taco as that's the other board that allows for good Ethernet I/O and more than 4 SATA, but I ended up just getting an entire CPU/RAM/ATX Mobo instead.
Maybe there's something hacky you can do to split the SATA ports in half to get up to 10 SATA at the cost of speed, but that doesn't feel right either.
I feel like we are so close to dirt cheap yet high performance NAS with these carrier boards. If only there were RPi compute modules with ECC RAM.
You might find some boards using an ASMedia ASM1184e 1->4 PCIe mux. There are riser boards available that just have this one chip on them and let you plug in four PCIe SATA cards. Those cards often use something like a JMicron JMB585 chip to convert one PCIe lane into 5 SATA lanes. Boards like this embed, for example, chained Realtek RTL8367 Ethernet switches to offer more network ports.
It's true that when all your carrier board IO flows through a single PCIe 3.0 lane, you do end up with a bottleneck. You're going to be unable to saturate 10 SATA drives simultaneously. However, your data source is probably not /dev/random, it's more likely a single network client reading from a single SATA drive. Remember, this isn't a high-performance server, it's a low-power ARM SBC (now an MBC, once you stack a CM4 on a carrier under a PCIe switch with some PCIe-SATA cards...)