Widewine and similar DRM schemes pretty much require hardware assistance; the lower levels that do not will provide you with 720p, which is exactly what you are getting in Linux. For 4k, it requires tee application, or similar mechanism that's not in the reach of mere mortals.
The early bypass of widewine meant burning an nvidia shield (invalidating its keys) for each and every single rip.
Do the DRM schemes interact with HDCP at the hardware level? I know HDCP is necessary, but my understanding has always been that the decrypted video data is always available to the OS (at the kernel level) and the "requirement" of it being outputted only to an HDCP-enabled sink was purely done in software through layers of obfuscation?
Higher-resolution ones definitely do. That's why you only get Widevine L3 on PCs and Macs, which most content providers limit to 720p or below.
You need something else (like Apple's FairPlay or Microsoft PlayReady) beyond that, and these definitely check your HDCP version. I believe 4k output commonly requires HDCP 2.2.
FairPlay on macOS might be based on obfuscation still (there was an interesting article on that here some days ago), but high-resolution playback on Windows definitely does involve the GPU driver somehow.
The early bypass of widewine meant burning an nvidia shield (invalidating its keys) for each and every single rip.