This is exactly my use case, while having a Tivo Roamio still hooked up that has sat unused for a few years.
Plex and Channels Plus both installed on my Synology DS1019+
Channel Plus handles my recording needs for anything I simply want to time shift, while Plex houses my permanent commercial free content that I acquire through other means.
Plex shines brightly in it's ability to house and display my permanent collection, while Channels Plus is both phenomenal and heads and shoulders above Plex in the DVR department.
While I don't point Channels Plus to my Plex library, I do have an extra Plex TV Show library pointed at my TV Show recordings in Channels, so that my friends and family can also enjoy that temporary collection too.
From a functionality & reliability point of view, Plex can't compete in the DVR & Live TV areas with Channels Plus.
TV Everywhere and playlist source integration in Channels Plus are two extra cherries sitting right on top of an excellent DVR engine.