As for the contender of the terrestrial planet with the thickest atmosphere, I have found a planet with an atmospheric pressure of over 8410 times of that of Earth
Step 1: Open locations menu (defaults to F6) Step 2: Click the "Add" button Step 3: Click "Paste URL from clipboard" Step 4: Press "Enter" to add it in Step 5: Enjoy!
(Feel free to copy and paste this wherever you need it, I made it myself)