Mouse control is pretty shit. I know the newer model has improvements, and you'll find Youtubers who claim it has the elegance and precision of a bit of medical equipment. The pads are pretty bad, and the haptic response is interesting but doesn't really improve them much beyond novelty options (or navigating the full UNIX OS). I do think if there is a use case for them it is in turn based strategy games, or other low threat mouse based genres, but I almost never use them. YMMV, as I say there are plenty of people who think they're great (they aren't).
Getting Epic and GOG working reliably and flawlessly, even after OS updates isn't the breeze it's made out to be. Yes, it's not too bad making them work, but it's a fiddle and really nowhere near as seamless as just buying the same games in Steam. I think the key issue is that it'll invariably stop working after an update of something or other and you'll have to fuck about. And then you'll need to work out if it's the OS update, the compatibility layer, Epic/GOG or something else that's fucked up.
Gamepass isn't worth the hassle - you'd be best just streaming stuff to it via a browser hack (but then you can use a phone for that). Yes, you can dual partition with Windows, or just about crowbar it on, but I'd say it's well beyond the effort I'd put in (but then I do have a large native Steam library).
Games that are good on deck fall into a sort of Venn diagram like you say.
Do they work well on pad controls.
Are they compatible with SteamOS
Are they performant on the Deck
etc…
If this is your back door to Sony exclusives then yes, God of War, Ratchet and Spiderman all run. Ratchet I think is on the verge, or at least it was a few patches back, but it does run.
If you're looking at any modern graphically intense title you normally have a choice.
Switch everything off, FSR, upscaling (540p) - maybe hit 60fps.
OR
Look reasonable - hit 30 fps.
Since the addition of the 40fps mode, that was a bit of a gamechanger. 40fps really fees great, and you can limit the screen and refresh rate to a locked 40hz. What you quite often end up with is that this is the sweet spot at 720p. With ladles of FSR, graphic tweaks, and following a few guides you can normally drag some pretty impressive performance out of AAA games. IIRC GoW, Horizon and Spiderman all fall into the area you might be able to tweak them to a reasonably stable 40fps and not look like ass. The big challenging game at the moment is Baldur's Gate 3, which has improved a lot in patches but is too erratic to lock at anything above 30fps (and even then has shitty frametimes).
For indies and lighter games pre 2015 you'll almost always be able to hit 60fps. Which is a bit plus over the Switch. E.g. say you fancied playing the Bioshock Collection. Hobbled 30fps shitfest on Switch. 60fps, smooth as silk on Steam Deck. I'd say this is where it shines, the huge, cheap gaming library available on Steam and huge volume of games from the modern 2008 era onwards (to about 2017) that run brilliantly on it.
Ultimately though I'm not sure how much sense the Steam Deck has to someone without an existing Steam library. For sure Steam Sales and key resellers like cdkeys can let you build up a massive library quickly, and cheaply, but it's still an ecosystem buy in. If you already have that ecosystem on Switch, then it's not going to really blow you away. And if you have an investment in GamePass I'd consider a ROG Ally or something rather than a Steam Deck, and just use it as a Windows device.