Probably the best device to do mapping (and everything else) is just a dedicated smartphone. For phones, if you look at amazon/ebay or even retailers like bestbuy, you'll easily find new units from $20-$50 and for what you are looking for, the smaller the screen the better (and cheaper). At this price they are throw-away devices so forget cases and just stick a garmin compatible mount directly to the back of the phone (KOM make a good one with stick pad included) and get a garmin style mount of your choice -Ebay is full of them and the Chinese ones are fine. And as you're already doing, bring a spare battery of enough capacity to match your ride length.
One huge benefit of phones is the software of the phone and apps is constantly being updated and improved so they keep getting better with age. Many mapping apps work offline but I've been using komoot the last few weeks and I'm liking it more and more as it can map roads and trails as well and works well off-line. For turn-by-turn directions I found Osmand app to be good and it works offline as well.
In fact if phones have a drawback it's that there are too many options - battery packs, apps, settings, mounts and the phones themselves.