Take it from a guy with multiple displays on the bike...
- keep them up as high and forward looking as possible
- only look at it 1 second and while going straight and clear of vehicles
So are you looking more for a dash camera system that allows bonus live rear view, or really primarily want a rear view option with a bonus DVR & front camera?
It's hard to have both in a stand alone system (as I'm sure you know.)
Looking at it from a rear view "mirror" standpoint only:
There are gadgets out there that work (or have been adapted to work) similar to backup cameras for cars - shoot the rear video feed and send it to a display for you to use as a rear view mirror - without DVR ability.
There are also a few helmets, (some out for a while and others coming out Feb 2020) that have either an integrated camera like the CrossHelmet x1 or mirror system like Reevu MSX-1, that gives rear view inside the helmet - no external screens required. Allows you to see what's behind you regardless of how you move your head. Pricey though!
Reevu
CrossHelmet
It all adds to more tech on your bike that could distract - and there's already a lot on a police bike (looking at your pics)
There's also another DIY type option that would involve splicing into the rear camera feed and splitting it. Send it to the DVR and to a dedicated dash screen. I'm sure it's doable. No WiFi or Bluetooth etc. needed. Dedicated screen. Downside is the tinkering to get it wired, and the extra screen on the dash/handlebars.
For now I simply am adding a convex mirror on the dash (better rear wide side view - not great for directly behind me) and will keep on with the shoulder checks, while continuing to wait and see what comes up as new tech / options in the upcoming months and years. <shrug>