Can I tell my Mac to charge to 80% only?
Use osx_battery_charge_limit.
Follow the instructions on github:
# 1. Install command-line tools
xcode-select --install

# 2. Install Homebrew
ruby -e &quot;$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)&quot;

# 3. Install Python3
brew install python3

Alternatively, you can also get python3 from python.org.
Then, git clone --recursive https://github.com/DevNulPavel/osx_battery_charge_limit, cd osx_battery_charge_limit, and finally sudo python3 main.py -s 80
I think it might be better if you set it even lower, for example, 60% or 70%. Then, before you need to use it on battery for a long time, sudo python3 main.py -r. You can automate this switching between 80% and full using cron.
There is also a GUI version: https://github.com/godly-devotion/charge-limiter. Both work by setting the BCLM key in SMC to a custom value.