It now runs, and quite well on high settings with all environments. That is surprising, considering that even some other low-poly Unity games on similar settings don't run as well as your game. While I'd recommend a bit more time for gathering feedback, this is good to go for me at least!
OS: KDE neon User Edition 5.14 x86_64 (based on Ubuntu 18.04 LTS "Bionic Beaver")
CPU: Intel i5-6198DU (4) @ 2.800GHz
GPU: Intel HD Graphics 510
GPU: NVIDIA GeForce 920MX (used for testing)
NVIDIA driver version: 415.23
Memory: 8 GB
EDIT: There seems to be an issue that prevents the player from moving the camera around indefinitely. The game doesn't apparently lock the (invisible) mouse cursor to the center, leading to the movement being constrained by the desktop boundaries.