Hi! Thanks for replying!
After logging on i3, htop shows me that I have a process called /usr/lib/polkit-1/polkitd --no-debug. I checked with polkit-kde-agent by running /usr/lib/polkit-kde-authentication-agent-1 &. I started MySQL with the same previous command (sudo systemctl start mysql.service && sudo mysql -u root -p -h localhost -P 3306) and checked the databases (SHOW DATABASES;), which worked: it shows the database with its schemas; but when I open Workbench again, it shows the same error message; so, from what you said above, it sounds more like a Workbench DBus issue (although I have no idea about DBus haha)