I recently tried Kaitai for something like this. It worked for some nontrivial cases, but I felt like I kept getting bitten by syntax limitations. All the way through, I kept wondering if writing a one off utility might be faster. But it was possible to express variable structures, sizes and counts. I was using the syntax and the cmd line visualizer, but might have been more productive with their html/gui tooling.

http://kaitai.io/

I spent a long time looking for something similar to sift through binary files and found this:

https://www.sweetscape.com/010editor/

It lets you write "templates" in a weird hybrid syntax that looks a lot like a C header file but also allows control statements that can decode complicated data structures in a few lines of script. It looks to have very similar capabilities to GNU Poke, but with a GUI.

The poke creator seems to have a very dim view of 010.

Exactly. Much worse syntax choices. Would not want to spend my time with it, when I have much more with 101.

But it least the VM is much faster than 101

https://ide.kaitai.io/ has a yaml syntax, and its online editor has tons of examples.

I skimmed through the manual and from what i can tell this seems to be more of a language (with a REPL) with support for handling data streams (they call them I/O Spaces for some reason) than an editor. There does seem to be support for declaring types of variable length and it is one of the first examples:

    type Packet =
      struct
      {
        byte magic;
        uint<16> length;
        byte[length] data;
      }

It also seems to have optional fields using conditions, using the optional extended header in MP3 id3v2 tag as an example:

    struct ID3V2_Tag =
      struct
      {
        ID3V2_Hdr hdr;
        IDV2_Ext_Hdr ext_hdr if hdr.extended_hdr_present;
      }

(this is an alternative to another way it provides by using unions which work similar to C unions but each alternative also has conditions for when it is valid).

So at least from the above i think it should be possible to declare types that work with variable length and prefixes like that.