Are Wine apps ran isolated? Because we basically have no viruses on Linux, but I imagine running random of course legally obtained games may be very risky
No, the Linux filesystem is usually mounted as Z: in wine. Sandboxing through e.g. flatpak/bubblewrap with permissions set to only allow access to ~/Games should protect from many viruses.
Wine does not sandbox in any way at all. When run under Wine, a Windows app can do anything your user can. Wine does not (and cannot) stop a Windows app directly making native syscalls, messing with your files, altering your startup scripts, or doing other nasty things.
You need to use AppArmor, SELinux or some type of virtual machine if you want to properly sandbox Windows apps.
Note that the winetricks sandbox verb merely removes the desktop integration and Z: drive symlinks and is not a true sandbox. It protects against errors rather than malice. It's useful for, e.g., keeping games from saving their settings in random subdirectories of your home directory.
I'm impressed how well wine-wayland already works. Some games still have issues with mouse locking, at least Overwatch doesn't register the at all. But as they said, bugs will be fixed over time.
I have issues with game resolution, when using Wine-Wayland the only resolution available is the one already set for my display, which renders FSR completely useless. As well as this little cursor render bug that sticks a frozen windows cursor on the screen, though I've been able to work around this.
I'm on 9.0 staging and can use wine with Wayland, but not everything works, window bars etc look somewhat off and some games don't start at all, like Stardew Valley. Other games I tried failed to hide the cursor. Others worked just fine.
I'm assuming you probably already know since you're making WineGUI, but older versions of Office (particularly Office 2010 32bit) are pretty simple to get working. Just need to install some dependencies with Winetricks.