Reverse engineer/decompile DLL to C source code

Hi,

Due to a ransomware attack, my software repository and laptop hard drive got encrypted and I have lost access to the source code that I need to recover.

I have a 64 bit DLL that I need to get reversed back to a human-readable C source code. Original code was also written in C.

Fortunately, that's not a fully-fledged app, but just a DLL plugin to a stocks trading platform. I have all the docs regarding plugin file format, and some source code of interface exposed by that platform. So we don't work in the dark. I'll provide all of the details that I have that should be very helpful in the decompilation work.

I'm a coder by myself but not a C/ASM expert. I tried to recover code using Ghidra, but apparently, there are some decompilation problems and it generates warnings like "Could not reconcile some variable overlaps". Code produced by Ghidra can't be compiled so a bit of love from an experienced reverse engineering expert is needed.

I don't need all of the files, but just a few specific exported functions and subroutines used in that functions.
This code itself is a stocks trading platform indicator that is used to draw visual aids on the chart. All of the heavy work is done by a trading platform so it's just a few thousand lines of code used to fetch some historical data from the platform, process it, and return the output used by the chart rendering function.

I also need your help in some minor modifications of that code to skip one part of the logic that is no longer needed.

Additional info:
Source code was compiled with the following command: x86_64-w64-mingw32-g++ -D _WIN64 -U NOMINMAX -march=x86-64 -mtune=x86-64 -O2 -shared -static -static-libgcc -static-libstdc++ -s -fno-rtti -fno-exceptions -std=gnu++11 InputFile -o Output.dll -Wno-deprecated

You will be provided with:
- 64-bit Windows DLL file
- Link to the platform plugin API documentation
- Header file of the platform interface
- Name of the functions to revert

What I need to accept this project:
- A concise, human-readable, C source code of selected exported functions + all of the associated internal code with private functions and data types used/required to run by the exported functions
- Code needs to be compilable by a standard C compiler with no errors, producing a valid, working DLL that won't crash and will display an indicator on the chart exactly like the one from the original DLL file.
- Performance of the compiled file must be on par with the original one (10% deviation accepted)
- By a "human-readable" code I mean no mixture of byte decoding, pointer arithmetic, operations on a CPU registers, in-line assembler function calls, and other unreadable garbage generated by some of the open-source debuggers, but something that looks like a generic C source code

We start with one DLL file and if you are able to fulfill my requirements I'll have more similar tasks for you.

DM me for the details. Please include references from a similar projects that you have done in the past