I think you did the right thing returning the iMac. I wouldn't buy anything other then SSD any more.
1TB SSD should be fine.
I would then still purchase an external SSD as your project/render/scratch drive for all video related work.
My MBP 16 has a 1TB SSD but I rarely go over 100GB! All of my documents and applications live on the internal drive, but I use external SSDs for all video related work. I like to keep th erender/project file son a separate drive so the internal drive just needs to run the application alone - things seem to work better for me this way.
Avoid the Samsung X5. Its a very fast and capable drive - I actually did recommend it in my initial review - but the cooling is poor and it WILL throttle the speed right down to HDD speeds after some video editing.
Personally I use either a ThunderBlade V4 (although appreciate a SSD RAID is very expensive), or if you want to go portable then the Glyph Atom Pro - as fast as the X5 but does not throttle with use. The speed is sustained even when the drive warms up as the cooling seems MUCH better.
Ive been through a lot of drives, as you can see from my YouTube channel, but the ones Ive stuck with are the two I mention above - the ThunderBlade and the Glyph Atom Pro.
If on a budget then the Glyph AtomRAID SSD is also highly recommended, and plenty enough in terms speed for iMovie related work......
Entire playlist of external SSD reviews as well as docks and hubs you may find useful!
NAS Drives, External RAID Arrays, External Disk Drives, and getting the most of your external media - its all here!
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