DCRainmaker has somewhat detailed instructions on how to select, download and install the free OSM maps.
How to download free maps to your Garmin Edge 705/800/810/1000 & Touring
You will need an SD card on the unit, 16gig works perfect as the maps are not that large, data wise.
Other then that the 810 pretty much does what you want, except can be buggy. I have one, purchased in mid-April and have had it crash 3 times, so can be annoying. If it happens again I will explore returning and switching to a Garmin 520.
Note that I use it for pretty much what you described - pre-routing and downloading of created course, basic tracking and uploading to Connect of my rides on assorted bikes. I hope to use it for riding in Maine and Vermont next fall on vacation, following routes I've researched and downloaded to the device. On the occasions I've done this locally, it's worked perfectly. I do not use any external sensors or HR monitors, just basic tracking and route following. I do find the BlueTooth download of courses as well as tracking and uploading to Connect to work really well and am happy with this capability, which is why I got this as compared to an 800.
As to re-routing, the 810 and 1000 will do this better then any other Garmin cycle unit, as far as I can tell. If you take a detour, it'll tell you are off-course but won't make a suggestion other then to turn around to find the route. As far as I've seen it cannot do an internal suggested re-route of a course you've created and loaded to the unit and are then following. If you continue on the detour and head back to the route it will know this and will then continue navigating and TBT.
The OSM maps are easy enough to read to find your own way, or you can attempt stop the course navigation to attempt a navigation to a POI or map point. Note that the 3rd crash on my unit seemed to be caused by my stopping mid-ride on a course navigation. Seems the unit gets confused at that point and freezes.
As note on POI's and timely updates, I think Garmin sucks at this on every unit they sell for general use - car, cycle or whatever. It's not a feature I require so have no experience with it.