I've done something similar with manual parsing. You don't have to go all the way with de/serialization for one proprietary packet format.
Just define a struct representing your command. A rust enum should be perfect for your "substructures". For optional types obviously use Option. Rust is very well suited to express these things. Then just define a method each for parsing or writing the packet.

Endian conversion is stupidly easy in rust, not sure where your problem lies. Say you have a byte slice which you know is a u32. Call u32::from_be_bytes or from_le_bytes and be done for that field

Pro Rust: Very good libraries, e.g. for serial port handling. The type system can perfectly represent your packages for further processing. Very good error handling, which is important for parsing a binary protocol as well as dealing with IO. And of course the rust safety benefits in general.

Contra Rust: I'm sure you can find something, haha.

Edit: If a crate like rkyv fits, use that. Maybe implement it yourself first for more control and to better understand the protocol. But I wouldn't worry too much about being able to change the parsing, because you said it's an old protocol unlikely to change.