MYVIMRC environment variable does not affect what
vimrc file is used for startup. I think you may have misinterpreted this bit of documentation:
The $MYVIMRC environment variable is set to the file that was first found, unless $MYVIMRC was already set [...]
This does not mean that Vim will use the file found in the
MYVIMC environment variable if it is set. It just means it won't overwrite the environment variable with the location of the file it used if the environment variable already exists.
To manually specify which
vimrc file to use without using
-u, instead you can use the
VIMINIT environment variable to source the file you want to use:
export VIMINIT='source /c/Users/theonlygusti/My\ Project/.vimrc'
As an alternative, you could instead use an alias so you still use the
-u mechanism to specify the
vimrc file, but don't have to type it out every time. e.g. in bash:
alias vim="vim -u /c/Users/theonlygusti/My\ Project/.vimrc"