Submit a feature request to Apple to allow downloading/offloading iTunes videos to external USB storage. The download likely must be performed on the iOS device itself but being able to offload should be a big help.
Also, do you have Movies Anywhere linked to multiple providers? Most of my iTunes movies transfer over to Amazon and Vudu. Even if Apple doesn't allow USB offloading, somewhere down the lines, other providers might.
Anytime there's DRM, playback options are often limited. I rip from Blu-ray myself and convert. Infuse handles a lot of formats beautifully (even non-recompressed UHD Blu-ray MKV rips with original audio). On a 12.9" iPad, the higher resolution of UHD was more noticeable than expected and HDR is something one can easily appreciate even on the smaller Pro 10.5/11 (perhaps on Pro 9.7 as well with DCI-P3).
My current PCs are too slow for 4K. Maybe I'll build a new one capable of hardware-assisted decoding and encoding of 4K HEVC. For now, I'm just using full quality rips which are gigantic (50GB minimum). Surprised my wifi actually handled the streaming. I really need to get the USB3 CCK.