Storage Solution (Speed + Storage) all inside iMac 27" 5K (late 2015)

Hi! I'm looking for answers and suggestions about storage solution for my recently purchased iMac 27" 5K (late 2105) 3.2GHz 32GB Ram. The first owner did a storage upgrade from the original 1TB HDD to a 480GB Sandisk SSD Sdssda-480g, but he damage the screen in the process ;-(. I'll replace the screen soon so I want to do another storage upgrade that fit’s better for my needs in this process. No warranty issues as it's already lost it.

My experience about Storage Solution: I’ve been using an iMac 27” (mid 2011) 2.7Ghz 16GB ram With a 250GB SSD Samsung 840 EVO + a 4TB HDD Seagate Barracuda. Operations System and Program Files (Final Cut, Photoshop, Pages, etc..) on the SSD. Other files and libraries on the HDD. For Backup i’m using Time Machine with a WD My Cloud 4TB Nas Drive. I also have a QNAP TS-453A with 2 6TB Red HDD (RAID 1) as a Plex Server, storage for movies, series, my personal video files and other old files backup.

My needs: I have large library files storage at the mid 2011 iMac (1.2TB Apple Photos / 1.0TB Final Cut / 1.0TB Lightroom), so I need good storage capacity for this most used files / libraries, and I would like really fast speed for basic operations (Operational Systems, Programs, etc…). Particularly I don’t like external devices for important stuff, as this library’s I’ve mentioned, and I also have a limited workspace on my desk and would like to keep all this “primary” storage devices inside the iMac, but also would like to have good speed as I will edit videos on Final Cut (Just to be clear about the amount of speed I’m talking about, i’ll edit 4K videos, and I would like to have decent speed for Final Cut operation (don’t need blazing fast rendering speed… I have time)

First Storage Solution Idea: Install a SSD Blade (250 or 500) plus a 4TB HDD. (O.S + programs SSD / libraries and files on the HDD, as no library will fit the SSD space)

Questions:

-Which SSD Blade model’s I should go for? I live in Brazil, and I can find some used blade’s on the market. Most of them used parts from MacBooks, with is cheaper and easier to find then new ones.

-Does the HDD has enough speed for 4K Final Cut editing operation? Like time line playback, background rendering, etc (again, don’t need blazing fast speed, SSD Raid 0, just looking for decent operational speed). 7.200 rpm HDD helps?

-If anyone have a better idea please help me ;-)

Backup: As I mentioned before I’ve been using iMac Time Machine with a WD My Cloud 4TB for the files stored on the iMac (Photos Library + Final Cut Library + Lightroom Library and other most used files as documents, etc..) I don’t know if this is the best solution, as when I’m editing videos on Final Cut the Time Machine backups lot’s of unnecessary temporary files. I’m thinking in Add a 6TB HDD on the TS-453A QNAP (4 bay, only using 2 bays with 2 6TB HDD Raid 1) for Time Machine and retire the WD My Cloud. This is a good backup solution? Should I stay using Time Machine or anyone know a better one with this equipments?

Thanks a lot!