Something like a Samsung X5 will nearly match the speeds of the internal - a T7 (~1000B/s) will not. But whether you need an X5 (and can justify the cost) for your work is another matter. For LR it is easy to keep catalogues on internal and photos on external - my external is a T5 (~500MB/s) . For FCPX you will need to do your own analysis - or someone else might comment.
Yeah, I also found this article quite useful:
https://fcp.co/hardware-and-software/2471-iain-anderson-on-ssds
Aside from external preassembled SSD's like T5, T7, and X5, I'm also trying to evaluate Thunderbolt 3 (maybe 4) enclosure solutions with internal SSDs. The Samsung 980 Pro looks insane at 7000 MB/s read and 5100 MB/s write. Something like that wouldn't get outdated for years to come with almost editing setup.
I've been thinking about this with current Macs that I have in mind, but if I do end up spending some money on it, then I'd also like to have a decent solution that scales well with future machines in mind. I've been waiting for the updated Apple Silicon 27" models.
Either I'll stay on the cheaper side for the time being just to meet my needs and workflow, or I'll go all in. Based on what I'm reading and seeing based on specs, some of the midrange to mid-high end options do charge quite the premium but still aren't as reflective in the performance.
Let the Black Friday countdown commence.