Question M.2 to PCIe 3.0 x4 shared bandwidth with PCIe 4.0 GPU

Aug 15, 2020
I am planning to build a PC with 7-10 years lifespan.
Graphics card and hard drives will be renewed every 3-4 years, rest will be the same.
I came to conclusion that to be able to upgrade my graphics card and SSDs for such a long period of time PCIe 4.0 is a must.

Motherboard: Asus ROG Crosshair VIII Hero (WI-FI)
Graphics card: Asus ROG-STRIX-RTX2070S-O8G
CPU AMD Ryzen 3950x
DDR4 64Gb

I have two existing 1Tb M.2 SSDs and plan to buy one more to have 3 M.2 SSDs in total.
The problem is that Asus ROG Crosshair VIII Hero has only two M.2 slots.
So, I decided to buy “M.2 SSD to PCIe Express 3.0 x4” adapter and put my third SSD to third PCIe 4.0 x16 slot.
My question is, what impact will have M.2 SSD located in third PCIe 4.0 x16 slot to my graphics card bandwidth? Isn’t it so that PCIe slots share bandwidth between each other?
What will happen in 6 years, when I will update to PCIe 4.0 compatible high end graphics card?