According to the Western Digital data sheet for your SSD, the amount of power it uses depends on the size of the drive; 256 and 512 GB drives are rated at 50 mW and the 1 and 2 TB drives are both rated at 65 mW of power used.
[link|https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/product/internal-drives/pc-sn740-nvme-ssd/product-brief-pc-sn740-nvme-ssd.pdf|Product Brief: Western Digital PC SN740 NVMe™ SSD|new_window=true]
However, when you go pull up the data sheets for the Micron SSD's being sold by iFixit for the Steam Deck, you get a much different story.
[product|IF107-162|Micron 2400 NVMe PCIe Gen4 2230|new_window=true]
[link|https://media-www.micron.com/-/media/client/global/documents/products/product-flyer/2400_ssd_product_brief.pdf?la=en&rev=94dd6337e2f643619508662720b6af62|Micron 2400 SSD With NVMe Product Brief|new_window=true]
According to them, all three of their drives, 512 GB, and 1 and 2 TB, are rated at "<200 mW" of power. That's saying that on average, the Micron drives are going to pull four times as much power as the WD drives.
So no, there's no point in going to the Micron SSD if it's power savings you're looking for; you'd be going in exactly the wrong direction.