After unplugging/rebooting/waking, regardless of whether the power cord is plugged in, the screen on my MBP (15-inch, 2017, MacOS 10.13.2 [17C88]) will default to a brightness around 80%. Is there any way I can disable this, or at least change the default to something brighter?
That's not a standard macOS feature nor a feature of the laptop. Perhaps something's wrong with the SMC? Display backlight problems can be indicative of an SMC error.
The display backlight doesn't respond correctly to ambient light changes.
You can reset the SMC. Shut down your Mac, then…
After your Mac shuts down, press Shift-Control-Option on the left side of the built-in keyboard, then press the power button at the same time. Hold these keys and the power button for 10 seconds.
The dimming when unplugging power can be controlled in System Preferences → Energy Saver → Battery: