What’s the best way to bail on the fusion and max out SSD performance?

So, my issue is that I really just want to speed up my computer. I am a producer and I also design and edit film on the side. I recently upgraded my 16 GB of RAM to 48 GB in my 2015 iMac 27 inch. It’s helped quite a bit. However, I’m still hearing system overloads within logic and Final Cut, and I am hoping to squash that.

Unfortunately, the first road I went down was thunderbolt external drives. And since the 2015 iMac maxes out at thunderbolt two, spent a good couple of hundred dollars on cables in converters and enclosures to try to get the best performance out of an external boot drive. The first road I went down was thunderbolt external drives. And it seems to me that at this point it’s probably worth just opening up my Mac, and maxing out what I can internally, even though it’s gonna be a ##&&% to get to the bottom of that motherboard.

So, my question is whether I can use the current flash drive converted to a 1 TB Nvme SSD, and disable the active fusion drive with the spinning 1 TB drive. Basically just using the internal HDD as storage and back up, and using this new one terabyte SSD as my boot drive along with everything else. Based on research, it seems that I can hit much higher read and write speeds and overall performance by going this route. And correct me if I’m wrong, but it seems like I can hit up to 3000 MB per second read and write speeds if I get the proper adapter and SSD. Let me know your thoughts, cheers!