Your best bet currently is probably to wait for the rumored upcoming Nexus 7 2016 revision, since you want a tablet that will stay updated and your current Nexus 7 (2013) did, in fact, get updated to Android 6 Marshmallow after all.
Edit: As it turns out you already own the 2013 revision; the second segment has been left for posterity.
Since you aren't requiring an e-paper display what you're effectively asking for is the 2013 revision of the Nexus 7.
- The Nexus 7 (2013 model) supports Android 6 Marshmallow officially.
- Both the Wifi and 3G models come in at 290g and 299g respectively.
- It lists for well under $250 now, having launched at $229 for the 16GB model.
- It's definitely the size of a Nexus 7.
- The base model starts at 16GB and there is a 32GB model available.
- It runs stock android since it's a Nexus device.
- This is the only requirement it doesn't make, and frankly, if you want real water resistance you're going to need a case, and that adds weight and size to any tablet. It can also add grippy texture or a strap which is something to keep in mind.
- This is a matter of opinion, but based on the size and rubberized type material of the back it sounds like many people can one hand it.
The only issue I see is that it's unclear whether the Nexus 7 (2013) will get an official OTA update for Android 7 Nougat, but there is some evidence that it might. You may also want to keep in mind that there are rumors of a new Nexus 7 releasing this year alongside the Nougat update and new Nexus phones. It might be a more compelling model and might also drop the prices of the older Nexus tablets like the 7's.
You're not going to get any closer to a stock experience than the Nexus line unless you're willing to dabble with ROMs.