Source multiple files in background/parallel (ZSH)

For various reasons I would like to be able to source a series of files in background and wait for them all to finish:

source ./my_file_1.sh &
source ./my_file_2.sh &
...
# etc

wait

But, while this does sort of work, they are sourced in sub-shells instead of the current shell’s context:

my_file_1.sh:

foo_bar() {
   echo "hi"
}

main_source_file.sh:

source ./my_file_1.sh &

wait

# zsh: command not found: foo_bar
foo_bar

Is there a way to source files in the background or in parallel while still having them modify the current shell context?