this will be used in the main slot or the spare slot?
Based on PSREF https://psref.lenovo.com/Product/Legion/Len...gion_5_15ARH05H
your machine seems to be somewhere along the lines of
512GB SSD M.2 2242 PCIe 3.0x4 NVMe + Empty M.2 2280 PCIe 3.0x2 SSD Slot

https://www.kingston.com/en/ssd/ssd-faq based on the theoretical speeds per slot
Using the new SSD in main slot PCIe Gen 3 x4 lanes of up to 4000MB/s. - can look for faster SSD like Adata SX8200, WD SN750
But if using in spare slot - Gen 3 x2 lanes of up to 2000MB/s. - would be a bit of a waste to use high end SSD like driving Ferrari in a school zone, speed is limited. Can consider WD SN550 or SX6000PRO

On second thought, think the main slot 2242 size is not going to really fit in for many of the aftermarket SSD (they tend to come in 2280 size) so I guess just getting a second SSD for the spare slot (2280 slot) get a midrange one will suffice. Even if put the best SSD inside, the PCIE Gen3 x2 will bottleneck high end SSD anyway.

Kingston A2000 or WD SN550. Thermal pad not necessary for both SSDs

Would welcome you to join Legion owners lounge: https://t.me/joinchat/Tsqm1x2osp1AaHgZy8XC3A

if you have the cash to spend then yes, however there are many good yet cheaper alternatives, and you can get PM981a (basically same SSD) from lenovo directly for cheaper price (SSD lottery)

Kinda overkill (secondary slot is speed limited), this 970 is like Ferrari in carpark (wasted potential).

Getting a midrange maivee will be good enough for a slot that is max 80kmh..regardless of whether you use maivee or Ferrari you won't go faster than 80kmh.
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For the 970, if you put it on a fast slot (autobahn) then only you will feel the difference.