Uh... Larian have actually gone native M1 + Metal so.... if they can do it at "very very low cost" anybody can.
No. Let me put it plain and simple and maybe a little exaggerated, because I'm a little tired discussing things over and over again with people who never worked in the industry and have done such things. You want your games and try to justify anything with wishful thinking, which is ok too. So sorry, but...
What Larian is doing is childs play, they're using a fraction of complexity of typical AAA games, so things are "easy".
Or in other words, how difficult can it be to port a C64/Amiga game to M1+Metal? (again exaggerated)
I don't think a lot of people realise how conceptually similar DX12, Vulkan and Metal actually are vs. the bad old days of OpenGL vs. DirectX9 or whatever. Which makes porting the small part of the engine to do rendering relatively easy vs. past APIs.
So since you figured out what a whole industry can't, let's start at the very beginning and let us know how you handle the memory residency management, driver state tracking, mapped memory synchronisation for CPU/GPU, lifetime management for objects and the binding model of a D3D12 as a whole in Metal? The argument buffers in Metal or obviously not capable of handling all this 1:1, so I keep wondering how "it's easy do to" works, unless of course one uses a very small subset of DX12 capabilities that matches that of Metal. Or are you using compute and issue draw calls manually? The other option is to start from scratch.
If all of this is so easy, I have to wonder why many assets/resources sold ready for use for engines like Unity and Unreal are platform specific. Could you try to run ML-Agents in Unity (again, click of a button?) on Windows, Linux and macOS and report back? It's "multi-platform" but on macOS broken (proxy communication). I gave up macOS support for one of my Unity based projects because of it. It's just not worth the time investment.
And BG3 doesn't look "dated". It looks pretty damn good actually.
I used "dated" to make it sound polite. We seem to have different definitions of how things look and your view also differs a little from that of typical "gamers". Let me say it again... bluntly, it looks like crap.
That doesn't matter though, because the focus of this game is not on looks, people won't buy it for that. When a game doesn't even have properly working shadows, then something is off by a mile. Again, people are not buying it for that, but they pay attention to it for many, many AAA games. Light-source <-> shadows. So yes, stuff like that is "easy" to port, because it's very, very basic.