Any of the latest generation Arm SBCs is actually pretty adequate for NAS purposes, especially if that's all you want to run on it.
If you get a Pi 4 or Pi 5, or one of the Rockchip boards with RK3566 or RK3588 (the latter is much more pricey, but can get gigabit-plus speeds), you can either attach a USB hard drive or SSD, or with most of them now you could add on an M.2 drive or an adapter for SATA hard drives/SSDs, and even do RAID over 1 Gbps or sometimes 2.5 Gbps with no issue.
Some people choose to run OpenMediaVault (which is fine), though I have my NASes set up using Ansible + ZFS running on bare Debian, as it's simpler for me to manage that way: https://github.com/geerlingguy/arm-nas
I would go with Radxa or maybe Libre Computer if you're not going the Raspberry Pi route, they both have images for their latest boards that are decent, though I almost always have issues with HDMI output, so be prepared to set things up over SSH or serial console.
If you get a Pi 4 or Pi 5, or one of the Rockchip boards with RK3566 or RK3588 (the latter is much more pricey, but can get gigabit-plus speeds), you can either attach a USB hard drive or SSD, or with most of them now you could add on an M.2 drive or an adapter for SATA hard drives/SSDs, and even do RAID over 1 Gbps or sometimes 2.5 Gbps with no issue.
Some people choose to run OpenMediaVault (which is fine), though I have my NASes set up using Ansible + ZFS running on bare Debian, as it's simpler for me to manage that way: https://github.com/geerlingguy/arm-nas
I would go with Radxa or maybe Libre Computer if you're not going the Raspberry Pi route, they both have images for their latest boards that are decent, though I almost always have issues with HDMI output, so be prepared to set things up over SSH or serial console.