A small form factor, if a prebuilt, makes a significant upgrade extremely difficult; you'll need to give further details about sizing and whether you can use a double-slot GPU.
There are very few low power GPUs in the recent generations. And why specifically 6 GB of VRAM? That's going to be difficult as well as most options for a prebuilt are 4 GB of VRAM. You'd also need specific PSU information.
Why specifically AVX2? This would be supported by anything since Haswell CPUs for Intel and all Zen CPUs (and Excavator before Zen, but buying a pre-Zen APU isn't an upgrade).
Basically, a lot more information is needed, including budget.