What is your veredict after one year?
all positive, 5 stars... well worth the money.
1) Motivation to learn Unity and try to create some app in the CAD/CAM field, not games, but rather immersive visualization of 3D CAD models and/or assembly instructions, etc.
BigScreen (App) has feature where you can make your own meeting room to present your CAD models in 3D to real people out there. i've not explore this as i dont have 3D model to present nor do i have any interest. so far i only love to go to virtual cinema to hang out, to watch movies or listen to peoples conversations in there.
2) Playing games like GTA V - question: do standard PC based games provide compatibility with Oculus Quest 2? Are special patches required? Would those patches be offical or void any membership?
only games that programmed to be able to play in VR, many of Steam Games i think, i only played DCS World and Ms Flight Simulator because thats why i bought the Oculus for. in Win10 you need Windows Mixed Reality feature installed, and then install the Oculus App for PC to make your Oculus device blended in Windows environment. you can google how to everything when you got your Oculus.
3) Exploring virtual worlds, including Google Earth and eventually MS Flight Simulator or similar
thats what the device is for. except i didnt see Google Earth App for 3D in the VR "Playstore". i got an app installed to explore ISS, control robotic arm and float in space in space suit. i wish there is such an App where we can walk in the virtual/futuristic city with real people everywhere, but so far none, the closest i can get to mix and talk realtime to real people in VR is the BigScreen App in confined spaces like cinema, room or balcony and i'm represented by a very simple Avatar which is not look like me

and the number of people is limited like 15 people at one time. there is another game i forgot the name, we play like in Tron + Ender's game environment where we can fly and throw disc to people (real or sim) and into goal ring (think of Quidditch game in Harry Potter but much more futuristic), but its been a while since i played it, today my kids are playing with the device. i got bored last time since i cant pass the landing training on USS Carrier in F15 jet, there is "trimming" control that i missed in the cockpit, that i have to do manual trimming by joystick, its almost impossible to make perfect landing

maybe next time i'll try again.
there is another App with natural open space VR world environment where we can fly like superman, but to get its full feature fly everywhere, we need to pay, the free version i'm stuck in the training facility. if you are willing to pay, you can fly everywhere. i think this VR is also good for theraphy if we are afraid of height like me. the first time i flew F15, looking around, i got the same nerve sensation when i'm in a real plane or high rise building/bridge.
4) Are the built-in/installable apps worth the trouble? Are they any good? Should I get a 128GB or 256GB version or is the 64GB more than enough (because the apps suck anyway)? I am looking for virtual representation of the world, not some stupid 3D block game
the way in there is like android smartphone, but in 3D mode where app screen is floating and you can browse the playstore to install app, there are paid apps there are free, i only interested in free apps and for my taste they are adequately good for learning and enjoying VR experience. if you want the next unlimited level, you pay for apps. storage is depend on your enthusiasm, if you like to play and try all of games, 128 or 256GB is for you. but if you want cheap arse get the minimum 64GB and uninstall apps that you got bored to make room for new apps. i think mine is either 64 or 128GB, lots of Apps/games install already and there are still free spaces. big app like the ISS simulated enviroment is 4-10GB size iirc. but other simple games only few hundreds MB to 2GB size.
and also please note, this device need to be linked (registered) to your FB/Meta account the first time you use it, if that matters. only few steps for registration and you are in. just remember dont do anything foolish in your FB account that makes FB delete you account, if your FB account got blocked/banned, your Oculus device will become useless too, thats the last report from people i read. hope it helps, cheers..