Gaming

Expedition 33

I haven't tried this game on the Deck yet, so I can't confirm these work, but have you looked at how other people have achieved 30 fps on protondb?

https://www.protondb.com/app/1903340?device=steamDeck

Another option would be to lower difficulty, if you haven't already. IIRC, the parry window is much wider on easy.

The friends we asked also gave split opinions

Classic too many cooks problem. Best stick to one person who seems like they know what they are talking about. Don't look at me, I'm just pretending. ;)

but I’m not sure about ventilation then

The more, the merrier. Map out a clear path for the air to travel through the device, from the least hot to the hottest component and out again. Larger fans are quieter at the same amount of air moved than smaller ones. Cable management gets more important the smaller the case is, so make sure to pick a modular power supply and tuck everything that can be tucked away neatly out of sight and out of the way of the airflow.

Without SteamOS it seems I’d need a keyboard/mouse out for every boot too.

You can configure Steam to immediately launch in Big Picture mode on any operating system it supports. There are many ways of achieving this, but here's a simple script for Windows:

https://github.com/jazir555/GamesDows

For couch PC gaming, I would also recommend getting one of (I have amassed several over the years, for some reason).

I’m also not knowledgable about specs, but I figured when, say, Borderlands 4 is not playable at 4K 60fps, then I could just deal with Full HD 30-40fps, which would be enough.

I would recommend waiting until it's out and people have thoroughly tested it. Within a few days of it reaching the hands of customers, there should be plenty of videos, articles and forum reports on how well it performs with all sorts of games, including particularly demanding ones like Borderlands 4. Digital Foundry are the best address for this kind of performance analysis.

The closest solution is PS5 Pro or PS6 when it comes out (and they fit on shelf, never checked size)

The PS5 (Pro or not) is absolutely humongous. I would be surprised if the PS6 ends up being any smaller. It'll probably fit onto your shelf, but not in it.

but, no Steam library then. Or mods, emulation. All in all, it seems I’m in an extremely specific situation where buying SM seems optimal.

Looks like it. This computer will undoubtedly be an emulation powerhouse, just like the Deck. If the price is right and if it doesn't have any teething issues at launch (which I doubt, given how mature the Deck was right away), then it might just be the ideal PC for you. If it's pricier than expected for the performance, then you might want to return to me and we can discuss a SFF build as an alternative, if you want.

One thing I would recommend is buying the base model and upgrading storage yourself, since this is likely where Valve will charge more than what it would cost to perform the upgrade yourself. There are no other spec differences between the models and it's likely that they'll sell the base model close to cost, like the Deck. Based on what I've seen, upgrading storage should be trivially easy, far easier than on the Deck.