Yeah, I remembered that LTT video also. Panned down through their vids and finally found it, only to realize it was the exact same product you were talking about.
That modularity comes at a cost though. You're paying a HUGE price premium just for the ability. Not to mention the cost of the modules themselves. And you're relying on Panasonic to continue their contract for modules far into the future, or else you're just stuck with a 1 or 2 generation dead end. For your budget, that modularity premium is going to net you something so anemic to start with that you're only going to be driven to spend more and more to get it up to spec.
IMO, it's a novel idea (one that's been tossed around before), but it's just not worth it considering the almost non-existent size of the market. I hate to encourage the throw-away-society, but you're just much better off going that route.
If you want modularity, get a desktop. Maybe an Intel NUC 9 Extreme if you want something SUPER tiny. Although there too, you're paying a premium for size and modularity that's going to exceed your budget.