Regarding the NAS; I totally agree. Pretty much the same reason I didn't go with one. Maybe it makes sense for folks who aren't comfortable with the Linux command line, but that's really the only time I would recommend a NAS over a regular computer of some sort.

As for speeds over USB, I've been using an external USB drive bay for a few years now and haven't had any issues, and I even stream 4k Blu-ray remuxes over the LAN from it. But it's worth mentioning that my enclosure supports USB 3.2 and wasn't exactly cheap. Link to the one I'm using is below: https://www.amazon.com/gp/aw/d/B07ND3JNZ6

Oh and regarding the taxing part: yeah, pihole is going to be pretty lightweight (it's designed for raspi after all). Sonarr and radarr aren't too bad either, but probably a bit heavier than pihole. You can certainly expect a lot of reading and writing from whatever your *arrs will use for downloading (nzb or torrent client). And I'm assuming you'll be using Plex or jellyfin, these will probably be heavier than any of the rest, again you'll have a lot of disk I/o activity, but even more so, if you end up needing to transcode your media. Plex/jellyfin by default does this on the fly and will be massively CPU intensive. UNLESS you're able to setup hardware transcoding. You'll want to check Plex/jellyfin's support for hardware transcoding and make sure you get a CPU that is supported by either or plan to get a supported GPU. In the case of Plex I think you'll need the Plex pass or whatever. Otherwise, you can just make sure your media and whatever device you're streaming to support direct playback, but I wouldn't rely too heavily on that, especially if you're sharing access with friends/family.