This is a simple-ish setup for launching BakkesMod alongside Rocket League and then killing BakkesMod when you exit Rocket League.
I don't use Windows anymore and I have no idea if the script actually works correctly! I'm posting this with the hope that someone might find the information here useful.
Download both bootstrap.bat and bakkes.bat. (You'll have to edit them and change filepaths depending on where you place them.)
In Steam go to Library and find Rocket League in the menu on the left. Right-click on it and select Properties. In the General tab you will see Launch options text field. Set that to "C:\path\to\bootstrap.bat" & %command% (you can put any other launch options before %command%)
Now whenever you launch Rocket League from Steam a BakkesMod will start alongside Rocket League. When you exit Rocket League simply wait until the process finishes and BakkesMod will close automatically.
Steam can launch a batch file through cmd.exe but just running the launch script (bakkes.bat) will cause the launch process to stall and Rocket League won't start at all. I fixed this with a bootstrap batch script that just launches the actual launch script in a separate hidden terminal.
The result of this is a terminal window that will pop-up whenever you start Rocket League. (It should minimize itself instantly.)
As far as I know there aren't many launch scripts for BakkesMod on Windows. This is one of few that also does opening and closing. It utilizes Windows Task Scheduler but the setup seems a bit more complex.