There is usually no real hardware difference between 30HT001PGE and 30HT001PMZ. The “German Windows” simply refers to the preinstalled language and keyboard bundle, and you can change Windows to English or French in the system settings.

I was able to do a search and according to the manufacturer's specifications, 1x 1TB SSD M.2 2280 PCIe® 4.0x4 Performance NVMe® Opal is installed. Below is the link to the exact same SSD: - https://www.digitec.ch/de/s1/product/lenovo-thinkpad-1tb-performance-pcie-gen4-nvme-opal2-m2-2280-ssd-iv-1000-gb-m2-2280-ssd-60093677 Otherwise, the manufacturer has specified the following mainboard: 5B21D18682 The mainboard has 2 additional Gen4 NVMe M.2 slots. If OPAL encryption is not a requirement, then 2x such an SSD can also be installed or an additional one: - https://www.digitec.ch/de/s1/product/samsung-990-pro-1000-gb-m2-2280-ssd-22097720

Yes, that is theoretically possible.

Yes, you can install an additional graphics card. The workstation has PCIe slots for optional dedicated graphics cards such as NVIDIA RTX models up to RTX 5000, but note that some configurations may have a support bracket in the chassis that could limit the installation depth of certain cards.

According to the manufacturer's specifications, this is a suitable optical drive for CD DVD: https://www.digitec.ch/de/s1/product/lenovo-ts-slim-90mm-dvd-burner-dvd-brenner-optisches-laufwerk-32451081