I installed Mac OS Mojave on the wrong partition. How do I go back?

Hey there,

In the past, I had messed up when trying to format my iMac Retina 5K Late 2014 and ended up with 2 different partitions for the two drives that are in the computer (one for the SSD, one for the HD) instead of 1 fusion drive. I installed Mac OS on the SSD, alongside my apps, etc, and was using the bigger HD for the rest of my files.

Yesterday, I wanted to install Mojave. The install selected by default the HD instead of the SSD as the disk for the installation, and I didn't think twice and pressed next. Once the installation was finished, it asked me to create a password, etc and that's when I realized my mistake. I now have two installs of Mac OS on two different partitions.

What should I do? I see two options:
- Backing up all my files to some external hard drives, and re-formating everything to have the original iMac setup (with a single fusion drive instead of 2 partitions for each drive). Then re-installing Mojave. I have no idea how to do all of that.

- Finding a way to boot on my SSD partition with High Sierra, where I have all of my apps and files, and erase the HD partition that has Mojave on it. Then install Mojave on the SSD. Not sure how to do that either.