HI EVERYONE!
Just wanted to post an update. I got everything installed and it works great! There was no need to split the fusion drive before installing everything. Once the HDD drive was pulled out and the SSD was installed it automatically split it from the Blade drive. A fresh copy of Mojave was installed on the 128GB Blade along with my applications/fonts, and everything else, including my user folder, is on the new 4TB SSD. Also, the tech who did this for me said you can't create a Fusion Drive between two SSD drives so that option was out. They also installed a heat sensor.
Now I have an extra 3TB HDD that was in the iMac originally. I might just use that for a redundant backup of my backups for anything important.
This 2017 iMac may be older, but it's blazing fast from what I was on last (2010 iMac with an SSD). 4K video projects that were taking 60min to render out now take 1min. AMAZING. The workflows seems the same, but the render time is amazing.
So now my specs are 2017 iMac 4.2 i7, Radeon Pro 580 8GB, 128GB Blade, 4TB SSD and 64GB of RAM (which I installed myself). Then I'll be running a 3TB RAID from two old SSD drives and now I have that extra 3TB HDD that I can use as a redundant backup. Spent a total of around 3K for everything. I could have bought a new 2019 iMac, but it would have cost me another 1K, which I don't have and this set-up should last me a good 5 years. I'm HAPPY! =)
Hope this info helps for anyone wanting to upgrade their old Fusion Drive machines!!!