8TB SSD is already here, you can get the Samsung 870 Evo 8TB SATA drive for $380. You can also find data center grade U.2 PCIe 3 Intel P4150 8TB for about $400 on eBay. There's also already consumer 16TB SATA SSD for able $1200 iirc
But you don't need this for 6TB of images. You can get a quality 10TB HDD for $190; get two for $380 and put them in RAID1. Done.
Going to such lengths to store all you images on flash with replication is not just inefficient but also doesn't offer any benefits. Even on a single spinning 7200rpm drive I doubt you'll ever experience slow down; SATA HDD speeds are normally 200MB/s, and standard raw photography files are on the order of 24-50MB, so you're looking at data loading speeds of a fraction of the second.
You're free to do what you like but the stipulations you present don't make any actual sense. You should be separating your "hot" fast working storage volume (NVMe SSD) from your "cold" mass storage volume (large HDD). Trying to combine high performance working volume with high storage capacity and data protection (replication, redundancy), all in a single storage system, is something that exists only in enterprise grade solutions ($$$$)
I'm also a photographer and this is the exact approach I take. Local laptop / desktop have only 1-2TB SSD storage, only for the photos I'm currently working with, all the rest get archived on a plain old RAID1 with 20TBx2 HDD's attached to a dedicated file server and accessed over the network as needed. Which is almost never because you don't actually ever need your entire photo library locally at once. Also utilize Adobe Lightroom CC (cloud edition) to maintain a library of all the work that I've edited, so even if you need an old file you can just dig it out of Lightroom easier than you could find it in mass storage archive.