Overall great experience with Proton on such an early build. Getting into the game took a few attempts, though this was in part due to an unstable internet connection. My steps were:

1. Wait for a 5-minute island in internet stability!
2. Launch the game from Steam, which will bring you to a secondary store / launcher.
3. If the store shows a message "Missing games from your library? ...", close and re-launch the game.
4. Create an account, or skip account creation. (I skipped without any repercussions.)
5. You should now be at a "Launch" screen, with the text "Installed from Steam". If not, close and re-launch a few more times.
6. Lunch!
7. First launch will require EULA confirmation - the window started minimised and I had to alt-tab over to it.
8. ???
9. Into space.

Some notes from playing around for a bit:

• I found that alt-tabbing is mostly stable - sometimes you don't get your controls back and need to re-alt-tab; but alt-tabbing is yet to cause a crash or other game-breaking scenario.
• The first time I reached the orbital screen and clicked through the tutorial dialog, the interface glitched out - planets were zoomed incorrectly and my controls were unable to control the camera. Clicking to a different interface and back solved this and it has not happened since.
• 1080p looked much more fuzzy than 2160p, where other games tend to do a good job of integer upscaling and crisping up the text.
• Graphics can look a little grainy or fuzzy sometimes - this is separate to the resolution point above. I'm assuming this is just some early-access textures & filters and will see improvements over time.