Consider:

On Linux, you'd only be fighting the Steam Launcher if things come to blows.

On Windows, you have to start fighting against the entire system, on day 1.

I wouldnt suggest windows purely from a performance standpoint. No matter what you buy you should use linux on it. Also windows isnt FOSS either so they could theoretically be doing the same thing.

Get a steam deck and either run games not through the steam launcher or just install another linux distro on it?

If you're concerned about SteamOS snitching because Valve I don't know why you'd consider Windows which has proven extensive telemetry.

Steam Deck easily. Everything you said you're worried about Valve possibly doing in the future goes double for Microsoft.

Steam Deck is a great platform for playing any games on, pirated included. I have my whole GBA ROM collection on my Deck and play them through RetroArch, works perfectly.

Steam / Valve doesn't care if you pirate games as long as you don't involve their servers. This is a pretty known / accepted fact.

If you really are worried just install a different OS on steam deck and don't use steam at all.

I don't think I would personally really consider using windows for anything these days. Semi-ironic to be concerned about steamOS spyware when you are comparing to windows which for sure has spyware.

You don't really need to use the launcher. You could run it from the steam desktop if you liked.

In my experience, windows is an awful UX for gaming but it is very easy to install and run unauthorized games. Steam deck requires a bit more heavy lifting and patience.

I ran windows on my deck for a year and switched back because the experience is terrible and nothing works quite right. I haven’t used the newer windows handhelds, but the upcoming ROG might fix some of the issues with booting into windows and doing constant updates

Use the Heroic Launcher. It uses the same proton as steam but doesn't launch through steam.

If your worry is that a corporation will report you for piracy, than you have a false dichotomy. Both SteamOS and especially modern Windows are both corpo controlled platforms. You'd be far better off running something like Bazzite off the handheld if the goal is to get away from closed-source systems. You also wouldn't have to run things through Steam to get non-steam games working, there's other options that would still allow you to use Proton as your compatibility layer.

Ive used it for 1.5 years with absolutely no issue. I made a separate steam account just in case, but honestly even if i couldn't use steam I'd still never put windows on there. Id much rather keep linux on it and just run games with lutris like people who have linux gaming pcs do. Steam makes it super easy and convenient but it isnt necessary for linux gaming.

Steam deck

Go with the Steam Deck and keep Linux on it. There are so many great ways to run non-Steam stuff from Steam if you really want to use Steam. And if not, just switch to desktop mode, it's just regular KDE and you can run whatever you want however you want.

Steam deck works very well for retrogaming and for running non-steam games. You can set up emulators rather easily.

It's an unlocked device (unlike a Nintendo), you run on it whatever OS you want. If they would pull such moves, community developed steam OS alternatives will arise. All that's needed to run non-steam games on the device is open source.

Given this hypothetical, I would rather Valve know I was a pirate than Microsoft.

Syeam deck is hardware. Afaik it's open.

Pick the hardware you like, install the distro you like.

Avoid nvidia intel windows.

Personally I find pirated games to be a bit of a pain to install on the Deck. I'd go with Windows for that use case.

Gaben doesn't care, he just is happy you spent alot on his handheld