Upgrading to SATA or NVMe SSD on iMac (late 2015) model

I have late 2015 27'' iMac with 1TB HDD, 24GB DRAM, 3.2 Intel i5 Core, and Radeon R9 M380 2GB. I have read that those models have an empty PCle connector inside. My goal is to speed up the system especially when I use the Adobe Suite. It seems like the HDD is the main bottleneck right now. To improve my iMac's performance I am considering two options.

1) Add a Samsung 970 EVO NVMe SSD with a ST-NGFF2013 OR THE ST-NGFF2013-C connector. Then move all the applications to the SSD while leaving the OS on the HDD. I have read that those NVMe drives are not ideal for boot drives. But here the question is would I see improved app performance, i.e. will i get the benefit of higher NVMe speed.

2) Upgrade the HDD to a regular Samsung SATA SSD.

I guess i want to figure out if I can keep the HDD for cold storage while utilize the NVMe SSD to boost application performance, assuming i move the Photoshop, Illustrator apps etc. to the SSD.

Thanks,