Both the new AS Macbook Air and Macbook Pro come with options for 512GB, 1TB and 2TB storage. Have you considered them?
I considered the 2TB option, for about 2 mins, then the price tag put me off. Going from 256GB to 2TB added €900, and between my wife and I, we have a collective total of over 2.5TB (and growing) of video and photo spanning 50 years, so we need externals anyway. We now each have an 8/256 M1 Air and can swap drives depending who wants to do what. Our 200GB iTunes library is also on an external. Only really apps and the OS on the local drives, you get used to externals. We then spend the money saved on things like vacations.
I'm busy merging multiple iPhoto libraries all into one mega Photos and one mega iMovie library, each on their own regular 4TB HD's, which cost just €90 a piece on Amazon. It'll be easy enough to upgrade/replace those if necessary. Those drives are then backed up with Time Machine, twice,
on separate drives which are then kept in 2 separate locations. Fires, floods etc, they happen. I live an hours drive from some of the Belgian villages that have just been washed away.
So far, the movie library is at 900GB, and iMovie isn't complaining. When the photos are done, that library will be about 1.4TB and contain over 150k images only as we are also extracting over 1000 video files from the Photos libraries. The idea going forward is to use Photos for photos and iMovie for movies. It seems logical, and you can scrub through videos with iMovie, among other things.
What I think is giving a few beach balls right now is spotlight, people, memories etc running in the background, but those activities seem to pause when you're actively using the apps. Otherwise, things seem to be working just fine, opening either the photo or movie libraries barely gives me time to take a sip of coffee. Browsing through them is fast.
We then leave everything on and connected when we go out, or to bed, so the background tasks can run. Those things will eventually finish, but we have a lot of stuff, so I expect it to take a while. But we do disconnect the drives sometimes, the background tasks then pick up where they left off when connected again.
You also don't need a dongle when using an external drive, just get one of these, from Amazon:
Also, keep in mind that Apple is now warning to not backup with TimeMachine a photo library on an external hard drive. See more at this link
https://support.apple.com/guide/photos/back-up-the-photos-library-pht6d60d10f/6.0/mac/11.0
If you read that carefully, they warn you not to keep a Photo library and a TM backup of that library on the s
ame external drive, because of possible permissions conflicts. I must admit, they could have worded that a bit better. It also defeats the object of backups if you keep them on the same medium as the original data.
Using Time Machine, it is perfectly fine to backup a Photos, or iMovie library, that is on an external. Just be sure to back those up on yet another separate drive. I'm doing that, I even did a test restore on a library, works perfectly. Just bare in mind you can only restore an entire library, not individual images within a library. The bigger the library, the longer it will take. By default, TM will ignore externals, so you have to set it to include them, if required. You don't want your TM to automatically back up someone else's drive.
You may be best served running huge libraries on external SSD's, if you're a pro and speed is a problem for you, and put Time Machine on traditional spinners which for the same price are much bigger than SSD's, TM drives don't need to be fast. TM can then just run in the background while you work.
Moral of the story, from where I'm sitting, there seems to be no official limit on the size of a Photos, or iMovie library, except for the size and performance specs of the drives you put them on. And, an M1 Air with 8 RAM will serve the average user just fine, and then some, no matter the size of library.
