I used Spans extensively. They help (mostly by providing efficient bounds checking), but not much. The main problem, as I mentioned initially, is the composability of this sort of code.
I wasn't trying to give some kind of defence of C++ here, just pointing out the dissimilarities to C#. This particular piece of code would have been far easier in C++ and RAII would have been a huge help, but the system as a whole would have been an utter nightmare in C++ (which I know because I've worked on multiple similar systems in C++).
Sure, but I am the opinion it does compose, if one embraces it isn't the same as writing C++ in C#, not trying to downplay your experience on the matter.
For example, here are the Roslyn analysers for a more RAII like experience with IDisposable,
Turn "DISP004-Don't ignore created IDisposable." into a compiler error and you have your region allocated RAII like experience.
And moving a bit the goal posts, for those scenarios where C# fails completly to provide a usefull solution, we can rewrite just that module into C++ (preferably C++/CLI when only Windows matters), and have the best of both languages.
I wasn't trying to give some kind of defence of C++ here, just pointing out the dissimilarities to C#. This particular piece of code would have been far easier in C++ and RAII would have been a huge help, but the system as a whole would have been an utter nightmare in C++ (which I know because I've worked on multiple similar systems in C++).