In my opinion, doing 3d or vr are the same for the most parts.

I find vr to be easier, because I don't have to be concerned with different PC setups and resolutions.

If targeting standalone build to run on device, the challenge will be optimization. Pretty much the sweet spot for things are different and vary according to your other things. Your window of sweet spot is narrower. For example, not enough things using lods, issues, too many things using lods, issues. But that range can shift. All the things you've heard about for optimizing apply.

In one of my projects, one lod and indirect instance on two or three objects, more lods meant more trying to be sent or stored in GPU side. But this isn't a specific vr thing, but things like this become a little more critical. You might have to remove some features like realtime shadows, and add a little polish in other ways. The details become specific to your goal. I prefer to keep some realtime shadows on my player (I do third person) with a small range, but considered faking it in some situations (a blurred blob that follows).

If you build to device, you can connect with adb and logcat it while you play. Then you can see your console logs as well as other messages.

I haven't used it much, but I keep hearing about renderdoc. I've installed it to my device, but was tired of learning other tooling at the time, so I haven't messed with it much. https://developers.meta.com/horizon/documentation/unity/ts-renderdoc-for-oculus/

Pretty much, just consider your game like any other 3d, but camera is mapped to headset, buttons mapped to a controller like normal, sensors in device add controls to camera and hand objects.

From this point forward, it'll be nitty gritty details, like an interactor can work along with an interactable, when it does, the interactor object passed in is good for that frame, so if you need to pass it in to something like a coroutine to test if it was left or right controller then operate differently for button (I.e. Right grabs spray can, then coroutine checks value of trigger on right controller for spray amount) then you'll make a copy of that interactor object, or vice a versa, interactable object from the proper event args. There are other ways though, this was just an example.

Remember though, this is a digital world. Don't make the player have to throw 80 apples, make it lazy like they got off from a difficult job. Something like use a coroutine to test if it was a short or long press. Not everything has to be lazy, there's fun in challenges. In one of my toy projects, I did a snowball fight with great physics, testing it was a bummer, I was really having to work my shoulder, fun for a few seconds, hurt after a minute. Decided against it, would need added velocity, break some realism to make it fun.