Which NAS or Motherboard should I buy?

Question

Requirements

I am looking for a NAS or motherboard with the following features:

1. It should hold 4 or 5 SATA 3.5" disks
2. 10 Gigabit Ethernet support
3. I want to use ZFS. The NAS should perform well with this file system. ECC memory is needed (8GB RAM should be enough I believe)
4. I want to install an Operating System manually, Debian for instance. My aim is to have more flexibility and freedom when I need to install new software.
5. I want to wake up the device remotely when it is on stand-by or powered off. I don't know if powering off the NAS is a good idea since NAS HDDs are thought to work all the time. I say this because synchronize my data once a week should be enough for me. If there is something very important that I want to backup I would wake the device remotely as I said before.
6. Low power consumption. I think less than 50W would be convenient
7. The computer's case should evacuate the heat efficiently and keep all the disks cold (30ºC). Passive cooling would be nice, but it's not a must.
8. I would like to spend around 100-150€ (or $) without disks. I can also take into account more expensive options if it's not enough

Solutions I considered

Synology and Qnap are good NAS brands, but they have their own operating system. I don't know if I can erase that system and install some Linux distribution in any of their devices, but I don't think so. I have checked that VMs or dockers can be used, but due to the tight resources I don't think that's a good idea.

• QNAP + Debian. There are some models where Debian can be installed, but they are old and with limited resources.

I believe that if I build it like a normal desktop computer with an ATX or Micro ATX board, the power consumption will be very high. I don't know if a Mini-ITX would fit my needs. Boxes for these kinds of boards are tiny and closed, but I would need one where all the disks can stay cool.

I found some options with Raspberry, but they didn't convince me. I don't know if it's going to perform well with ZFS. Anyway, what I like about Raspberry is that its power consumption is very low:

• Raspberry Pi 4 + Quad SATA Hat Board. A YT reviewer says that the box is not breathing well and the disks reach 50ºC of temperature, which I think it's too much. That could be solved by creating a new box with holes where all the 3.5" disks can be placed. They use USB 3.0 to plug the hat board and convert them to SATA ports, I don't like that solution because it's not very clean.

• Raspberry CM4 + PCI Express Board + PCI Express to SATA Board. I think this would be more expensive because I would need to buy two boards and the performance would be similar to the previous one.

• Raspberry CM4 + SATA Board. Wiretrustee is developing a new SATA board to work directly on the CM4. But it's not available yet.

• Raspberry Pi 4 + 2-bay DAS with USB 3.0 (there is a 5-bay DAS as well) + SSD (for de Operating System, SD cards are quite slow). I could take a 4-bay DAS, but I don't know if 4 HDD would be much for just one USB 3.0 port.