Nah, Diablo works fine on Linux.

I would stay away from Manjaro, it has a history or being unreliable due to the holding back updates for 2 weeks. EndeavourOS is getting recommended a lot recently, I’d go with that instead!

The EA launcher works just fine via Lutris. Source : I’m using it to play Mass Effect Legendary Edition perfectly fine.

Battle.net works as well, OW2 plays fine on Linux.

GOG & Epic work through Heroic Launcher as you mentioned.

As for a distro, I’d aim for something Arch based. Simply because you’ll get the latest drivers and mesa versions all the time, which is proven to increase FPS a ton over say, Ubuntu.