How can I find out what is upgradable?

In any laptop you yourself cannot physically replace the graphics card. The graphics chip are soldered into the motherboard and require special tools to remove it. Even then you cannot just buy the chip and if you can it most likely will not be compatible with the motherboard mounting section for the graphics chip. You can however buy a graphics card amplifier if your laptop can support it so check with the manufacture if you can do that. If your laptop supports it you can add a NVMe or PCIe SSD to your laptop as an extra fast boot storage. Otherwise all you can do it what you said above except replacing the graphics card. You will see performance gain from upgrading your graphics card but I have no idea by how much as your CPU might bottle neck the performance with superior graphics cards. Also you do not need to uprgade the optical drive cause in 2018 no one really uses it anymore so just leave it with the original.

You can upgrade the graphics card. You can upgrade to at least a gtx 980m. These are interchangeable mxm pcie format. The cost is pretty high though. Mxm cards tend to be about 20-30% higher than the normal full fixed pcie desktop graphics cards. You can find them on ebay or all over online.