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1) I develop SaaS. So websites or apps + the service that could be something running 24/7 but this would be another topic about hosting, I wouldn't host it on my daily driver obviously, and anyway these things can be easily "work stuff", things meant for the public, I don't care about it. No videogames for sure.
2) Which policies? Policies like the ones for browsers. Forcing browser settings. Can be done easily both with Chromium and Firefox based browsers as far as I know and doing. If anything available for the OS too, it wouldn't hurt (for example Windows Group Policies). But the fundamental thing for me is the browser policies and possibility of knowing which apps/processes are running on the pc. For example if I run a VM, the host OS can't know which apps are running inside the VM unless I also install something that monitors that on the VM and tells the host OS. If using VMs, I must be able to do that. I don't think QubesOS lets me do this. And anyway, no GPU no good.
3) Which apps? I will need to use the browser (Mullvad or Tor, I only use these two based on my use case, because they are the only browsers with good anti-fingerprinting). Then some IDE for developing, I guess. Some open source app/script for encryption, backups, sync. Video and image editing software, for example Photoshop and something to mount videos (basic stuff, I guess). Docker for something local. Maybe some small local AI. I don't care about video games because I don't play them and don't want to play them.
Messaging apps (Signal for example).
And.. I could need some other professional software I can't think of now. Some third party thing. Surely, something like Office for offline use, this is something I use as of now, mainly Microsoft Word without internet access.
4) What do I wish to conceal? I want to prevent an app or websites to know what I'm doing on other apps or websites.
5) The companies I want to conceal data from are any companies I didn't choose to give my data and just that selective data to. Big corporations like Google, Meta, Apple, Amazon, Microsoft, are the main threats for example. But also ads, more in general, and data brokers. I want to do share the minimum amount of data to the minimum amount of people without crazy usability inconveniences or daily fixing bugs/issues. For example I don't want Google to know that I've been on a site reading about some personal health issue or curiosity. That would happen just with google analytics on most websites if not using Mullvad or Tor. And if I login on Mullvad or Tor obviously they can know, but for example I can login on a website of a given domain, but that website cannot know I'm searching things on another websites because third-party cookies are blocked and I don't have unique fingerprint.
Anyway, device fingeprinting would be more difficult. On GrapheneOS I'm switching from only 2 profiles (owner and play services profile) to multiple profiles not running in background, and the Owner will use just open source apps or closed-source apps without Network permission. Then the multiple profiles will be useful for preventing IPC and querying all my installed apps, between certain groups of apps and compartimentalize more. I'm in the middle of doing this.
On desktop I don't know. AFAIK, this is only possible on Qubes, or if I use a VM for each app in general.
6) Obviously if I pay a company and give them my name they will know my name. They won't know my name if I use a virtual credit card just for that purchase and "Name Surname" as my name. And, anyway, I will obviously know that they know who I am and assume that. But, I don't want them to know at which time I go to the bathroom or search X or Y that isn't related to them. So, I miss your point.
7) I already read lots of things for months. I've got no problem on tradeoffs and already know what are my priorities and stated in the post above. For example I said that I care more about privacy, but above privacy I need usability for software developing and video/image editing, then the browser policies and the possibility of knowing which apps/processes are running on the OS, then privacy, then security.
I need an OS, a PC to choose, and then just move on with life and focus on work instead of privacy or security (I already spent lots of time on these topics and at a certain moment the returns exponentially decrease).
What I asked is feedback of practical use and experiences from people, and a double-check to know if I'm missing something. Having third party opinions. And, corrections, eventually.