iCloud is a terrible option for video editing assets because it is far slower than local drives. It doesn't matter if you have crazy fast broadband, iCloud servers themselves are slow, so that would be a relatively painful bottleneck for PRORES editing which does a huge amount of read/writes while you edit. I wouldn't even consider iCloud storage for any video editing at all myself. Instead, I'd wait until I can get to a more robust setup with local drives.
iCloud will definitely not solve this problem. I don't even bother with it for photos storage, iDevice backups, etc. In fact, I almost don't use it at all... except as a convenient way to quickly share (mostly) small files with other Macs or iDevices. In all things of size, local storage will be far faster. For video editing, I consider big local storage
essential... as I don't want to wait all that time for countless editing read/writes to do their thing with slow cloud storage, whether iCloud or any other.
Your overall posts read like
costs are the main driver of your frustration: having to pay for a video editor to output the resolution & fps you want to target, having to pay for local storage vs. using "free*" cloud storage, etc. However, in wanting to rise above what iMovie can do, you are stepping into a more professional needs level... and at that level, I doubt you can find anyone "pinching pennies" in supporting hardware and being happy with their video editing experience. If you don't own big local storage, 4K PRORES editing will be painfully slow using storage in the sky.
While it will take laying out some cash, local BIG storage is going to be VERY NOTICEABLY SUPERIOR to what you've apparently been doing leaning on iCloud storage. FCPX is going to be able to not only deliver what you want- 4K60fps output- but also much more beyond what is possible in a simple video editor like iMovie. Many pros use FCPX, so you will be shifting from hobbyist editor to beginner pro editor, using the very same tools they use. As such, you'll be gaining the same range of editing capabilities they have too (which are far greater than iMovie).
Yes, it will cost a fair amount of additional money- especially if you are wanting to archive editing projects instead of using the "edit to perfection, render 4K60fps master, then delete the project" approach- but that will get you on the path to faster editing, better editing, better output, flexility to do whatever you want (instead of only what an iMovie-like app will facilitate), etc. Yes, there is a bigger learning curve but you'll be taking the first steps towards learning to edit like professionals instead of hobbyists... shifting from- if you will- playing with toys to playing with power tools.
You are not the first to crash into this need and then step up to a new level. That new level removes many limitations (beyond only 4K60fps output) inherent in amateur editor apps like iMovie and similar. Your hardware stack is about to grow so you can do what you want to do here... and that supporting hardware will also help with many other computer uses too.
For anyone wanting to do it all within a single laptop and no supporting storage, they probably need to be the one's paying way too high for that 8TB SSD and BIG RAM in their Mac (probably with MAX Mx chip too)... and even that is going to prove to be only enough space for editing fairly short videos if 4K/60 PRORES is the working medium. As anyone opts for lower spec Macs (usually driven by price), those wanting to edit lots of video will then need lots of external hardware, particularly fast & big storage. Else, it will be a much slower editing experience waiting on slow distant servers to update over and over in all those read/writes... and much slower editing asking the app to work with compressed video during editing (which will almost certainly lead to overall slowdowns per system throttling) instead of raw PRORES (which will then make use of that external big storage), etc.
My own approach begins with converting video from whatever format it is shot in to PRORES files. They need a LOT of space just for those RAW PRORES files. Those files are then what gets imported into FCPX so that I'm only working with PRORES (no compressed format processing required) which yields maximum editing speed. Most editing of short videos are done on a pair of 8TB SSDs, one inside Mac and one in a Thunderbolt dock. When videos are bigger, I often bring a HDD-based RAID box into play (not as fast as SSD but pretty fast for bigger storage demands).
Edit files to perfection, export final creation as one big PRORES file (often 4K/60fps- just like OP), then run that through Handbrake to compress it down to h.265 with high quality settings for picture & sound. Carefully proof that final file. If it is "perfect", delete the editing project to free up the space for the next project. If I need to keep some editing project for further editing/evolution in the future, I move the whole project to gigantic archival storage in a Synology 12-bay bank of big HDDs.
I don't know if this is the best way to do all of that but it is FAST and works well for
all of my editing purposes.
Next supporting purchase is probably a big RAID-based SSD product like
OWC Thunderblade with 32TB of fast SSDs... or hopefully the newer Thunderblade X8 (8-Bay) one will be able to handle 64TB SSD in time. And if I need bigger fast storage for future products, add another of those and RAID BOTH
together for 64TB or 128TB of fast SSD storage.
