You can customize a almost everything using the Keyboard panel of System Preferences. Yes, 3rd party tools like Karabiner are needed for control of absolutely everything but that is pretty rare.

Also note that accessibility must be “unlocked” because it fundamentally adds a security risk: processes that can inspect inputs in arbitrary unknown applications have a lot of power. What macOS does is actually a feature and it prevents one of the “tire fires” in Windows.

Beach balls in my experience are usually produced by problems in drivers, or programs that are unnecessarily complex (like the ones that are 400 MB installs with entire virtualization layers instead of being all native code). The OS can only do so much.

External displays: yes, definitely buggy on Macs these days, and with some obscure settings. Something to try: with at least 3 desktops/Spaces defined (need to use "+" in Mission Control), you can right-click on the Dock icon of an application to specify the default space to use for windows in that app. These menu commands are not available with less than 3 Spaces created.