It's definitely not a bad idea to expose the camera to the Plugin API, but if you want to implement something like head bobbing, you will probably have to modify the camera every frame... unfortunately this wouldn't work well with the Plugin API, since it's designed to only run serverside (when thinking of it as a multiplayer game). So any changes done through the API will be applied to the client with a certain delay (typically the ping between client and server). Ofc there is no server when playing in singleplayer, but you still have a minor delay (typically a few frames), which makes any frame-critical operation impossible.
The API can only circumvent that by providing certain helper methods, e.g. something like a method to set a camera position/offset smoothly (so the game performs the smooth movement under the hood). This way you're still more limited compared to having direct access to the camera, but it should work in most cases. Not sure if that would help in this case?
Drawing outlines around elements is more difficult though, especially since the game is using Unitys HDRP. Passing custom shaders to the client is actually possible through the API, but the HDRP also requires you to write a custom pass to achieve such an effect. Unfortunately this isn't something that could be exposed to the API, even if the API would run clientside...
So that definitely requires classic modding. Currently we have no plans to switch to Mono unfortunately... we might reconsider this in the future, but we ran some tests back then and things like chunk generation were up to 6 times slower in Mono compared to IL2CPP...
However, there is an ongoing effort @Unity to switch to .NET CoreCLR (which would even provide better performance than IL2CPP), but unfortunately there is no ETA (and many people who were originally involved in this process no longer work at Unity, so I'm afraid this switch won't happen anytime soon)... 
However, BepInEx also supports IL2CPP games. Ofc it's more difficult to write mods (due to the lack of easily accessible source code), but it should be possible to create a mod for the game this way 