but it will be hard to play DRM protected media, eg Netflix on a device like this, right?

This is just a DE for Linux, it does not solve the problem that DRM is incompatible with a free platform. Get your content from another source than Netflix.

720p using widevine. I play it. It works. Even if I disable DRM in my main browser. And only isolate it to my Netflix account.

You can get 1080p on Linux with Opera https://help.netflix.com/en/node/30081

Weird, why just on opera? It uses the same engine (chromium) as many other browsers.

I'm guessing because of a higher widevine certification level or a server-side policy?

You can also spoof Opera's user agent and get 1080p on FF so guessing it's a server-side thing; since Linux has L3 widevine certification because no kernel level TEE

I have no issues on Firefox FWIW. I haven't needed to spoof the user agent, though this is something I needed to back in the day when they literally blocked FF's user agent.

Is that through a dedicated Netflix profile or is there a way to enable DRM per site?

the magic trick is just to not, netflix won't even stream you 4k 90% of the time even if you do have all the requirements.