Basically the only sticking point IMO would be whether the specific games you enjoy tend to have problems (often due to draconian DRM or anti cheat systems)
You are wrong in terminology. Capital gains tax is not the same as income tax. The “realized“ term you mentioned is already redundant with the “gains”. If it’s unrealized there is no gain.
You seem to be under the impression that these existing categories are the only kinds of tax that can exist. In reality, it’s entirely possible to structure taxes in any way that people can devise.
I’m not super-familiar with mod managers in general but my experience has been that they just work with proton more than you might expect. I know for sure that unity mod manager works fine in proton; I use it for railroader.
See also “corn” (maize)