I'd recommend SageTV. I've got an Nvidia Shield (16GB) on each one of my TV's with an HDHomerun prime as the tuner. I use a Windows 10 Pro 64-bit server as the server and the Nvidia devices as clients. You could also use Linux as the server OS and that might work even better. The big limitation with SageTV is that there's no DRM support. The only PC software up to this point which supported that is Windows Media Center. With Comcast, there's only one station in my lineup with DRM so it's not that big of a deal for me.
The basic setup is easy and you can basically just accept the defaults. You need to setup an external tuner for the HDHomerun Prime; I used OpenDCT since it seems to have the most support. If you aren't a previous SageTV customer, you also need a guide source. Schedules Direct is the most convenient but will cost $25/yr and is supported natively within the app. You'll have to use addons and do importing of XML for other options. You'll have to edit which channels you really want guide data for. I got rid of a lot of channels which I'll never watch in my lineup which were duplicates, foreign language, non-HD, etc. That takes some time but provides a cleaner guide in the end.
The other thing that takes some time is to figure out how you want to control the system. I suppose you could just use the game controller but I wouldn't recommend it. I wanted a traditional remote control. The IR sensor on the Nvidia Shield doesn't support third party remotes well. I'd recommend using a FLIRC; the FLIRC will map IR codes to keystrokes. You can then map the keystrokes to SageTV commands. I started off using a Logitech Harmony 650 remote control but am now using a Tivo Roamio Remote in IR mode. You can bypass the FLIRC if you spend more money on a remote compatible with the Logitech Harmony Smarthub. A remote like the Air Mouse which has its own USB receiver also eliminates the FLIRC. If you want a decent wireless keyboard for the Nvidia Shield, the Logitech K400 works well. I can do pretty much all that I need on the Tivo Remote/Flirc combo and like that because of the key layout and its durability.
I dropped Plex and am really just using SageTV for all DVR functions and media playback. This gets me the same UI for everything and it seems stable so far. Here's something that I don't like about it. The time from hitting the button to change a channel and seeing video for the new channel is longer than I'd like. It's typically 8-15 seconds. This is apparently an issue with all of the DVR software. If you just want to view and not record then you can get near instant channel changes with the HDHomerun app or the Channels app. The program's not perfect but it's my favorite so far on the Nvidia Shield.