Blender Remesh Error in Sculpt mode. How do I decrease the polygons correctly and save the project? I have a big problem.
Some time ago I asked a question about this project regarding the selection of polygons to be able to work in zones.
The problem now is another, not being an expert in polygonal sculpture, I immediately started creating my character at the maximum polygon resolution. For this reason now the file weighs too much and when I open it despite having a PC worthy of power, the 500k vertices and 900k faces I would say are really excessive.
In fact, every action that I go to perform in blender, whatever it is, needs time to load it.
In the previous question, I received an answer that my project had too many polygons and was advised to remesh my character, thus creating a low poly copy.
However, after some time I just tried to perform the remesh, perhaps inserting wrong values, I don't know, even if I took reference from the screenshot that was recommended to me.
Performing the remesh I encountered these problem.
Problems :

The whole surface is pierced, and my mesh is corrupted as shown in the figure.
even the color used with vertex paint is canceled and transformed into a totally reflective surface.

Principal Questions:

Am I still in time to be able to recover this project or is it to be thrown away and redone from scratch?
How can I consistently decrease the weight of my file without completely ruining the character?
Why does remesh create these holes? Can it be caused by vertex paint? How do I resolve?
(I also tried using the Decimate modifier too, but I think it's not the exact tool).



Extra questions:

What would be the right technique to create a character, what are the precautions to use, especially not to stumble into certain errors.
Above all what are the steps that should be followed for the complete realization of a character? Such as creating a low poly copy, how to color in the right way, but above all when to do one thing instead of another.
For these two points it is clear that a simple answer is not enough, so if you know where to send me to document me I would be grateful. (A site, a guide relating only to this). Or even some simple basic concepts that make me understand the steps to be respected.

p.s. After completing my application I was able to find several questions already asked, related to some of my extra doubts, so if you agree I will not delete these questions as maybe they can be used by someone who is looking for the same things and cannot find them. However, I have found several interesting answers for these extra questions of mine, which I will attach the links to my question. while obviously higher priority will have to be given to questions concerning my project. However, opinions, reflections and advice are always welcome.
For example this question, yes, can answer some of my doubts and I also found a video on how to paint in layers. Paint high-poly mesh and then bake for low-poly, OR just paint low-poly with normal-map applied?
But I think I need a specific answer to this question because I can't work on this project anymore. I want to resolve my fault.
While that other question has managed to answer some of the polygon sculpture questions about how it's best and what it's best to sculpt a certain way for.
What is the importance of low poly vs high poly?
-I tried to upload the file but it weighs 90mb, and the maximum is 30mb. how do i compress the file to upload it through the exchange? or should I use something else?
[ https://drive.google.com/file/d/1Idc8Jg9wtHwQey5V7qpwuxwtw1QjEM4K/view?usp=sharing ]
Thanks in advance.
I just wanted to add that I encountered the same choppy model result from remeshing and that closing up all holes in the model solved the problem.
This was the result using Remesh when I had forgotten to close the neck hole:

After closing up the neck hole the Remesh worked fine:

Happy Blending!