So it seems like Buster comes default with 2.7 on load and an alternate 3.7 version of Python. However, I am using this Raspberry Pi 3 B+ just for an application that is only compatible with Python 3.5. How do run Python 3.5 as default version or remove 3.7 entirely from Buster?
I truly believe that you can just install python3.5 from the source repos.
If you want to install python 3.5.4 : 
sudo apt-get install python3.5
If you want to install python 3.5.3 (Debian Stretch) :
Edit as root /etc/apt/source.list and add the following repos,
deb http://mirrordirector.raspbian.org/raspbian/ stretch main contrib non-free
rpi firmware deb http://archive.raspberrypi.org/debian/ stretch main ui
Then install the specific package version from the target repo using the following command : 
sudo apt-get install python3.5 -t stretch
If want to install pip3.5 (which i guess you would need) : 
wget https://bootstrap.pypa.io/get-pip.py
python3.5 ./get-pip.py
Downgrade Python 3.7 to 3.5 on Raspbian Buster