While there are laptops which accept 3 or even 4 SSD's, and which could do this task, those are very expensive, so few might have one handy.
Older ARM SBC's had many limitations in the number of PCIe, SATA and USB 3 interfaces.
Nevertheless, now a much better ARM CPU has become available for cheaper devices (i.e. in the $100 to $200 range), the Rockchip RK3588 (quadruple Cortex-A76 + quadruple Cortex-A55 + triple Cortex-M0).
For now, this is the only ARM CPU with a decent speed, comparable or better than the Intel Gemini Lake Refresh CPUs, except for the much more expensive NVIDIA Xavier or Orin, or the Qualcomm Snapdragon SBCs, which are much more expensive than Intel/AMD CPUs + motherboards, having similar features.
There are at least 5 or 6 companies who have announced boards with RK3588, ranging in size from credit-card size, like Raspberry Pi, to the larger picoITX, and up to the miniITX form factor. Hopefully such boards will be available in the second half of 2022.
Some of these boards have up to 4 SATA connectors, besides an M.2 NVME SSD slot, so they could be easily used as a NAS.
According to the published reviews of such a board, the typical total power consumption (without SSDs/HDDs) is around 5 W, and the peak power, at maximum CPU utilization, around 13 W.
There are a lot of different devices that include ARM cores.
Some of them have actually been designed for TV set top boxes as their primary application, e.g. most of the models from Rockchip. These support a lot of video formats in hardware, even more than typical desktop GPUs from the same year. For example the Rockchip 3588, launched this year and used in many single-board computers that have just been announced, is one of the first devices providing a fast hardware AV1 decoder, besides decoders and encoders for all older codecs.
However, some of these devices with ARM cores have vendor-provided video drivers only for Android, to be used in smart TVs, so it may happen that the device used in your Synology NAS actually supports in hardware more video formats than you can use, and you are limited by the available video driver, which is incomplete.
My home server is mainly my home NAS.
It would be hard to put 12TB of mirrored storage in a laptop.