That’s what GNU Parallel is for. Or Pueue, which gives you a GH-Actions-level feature set but it’s less likely to be installed on any particular machine. Pretty sure you could fetch a pueue binary at the start of an actions script and do everything that way.

Using GNU make you can get very nice output sync:

    all: variant-1 variant-2
        @echo done with all
    
    variant-%:
        @echo starting $@
        @sleep 1
        @echo done $@

Use `make -j2 --output-sync=target`.

    brew install make
    gmake -j2 --output-sync=target

on macOS.

>Yes now try waiting for all parallel tasks,

`wait`

>and error out if at least one errors.

`wait` propagates the exit status of the waited task. `set -e` triggers the script to exit on any command failure.

>And try separating their output so that they don't get interleaved into a big mess.

`| while read -r line; do echo "task X: $line"; done`

Learn the tools instead of being so confident that it can't be done or that it's complicated.

You remind me of that guy who said there's no point in Dropbox because one can "just simply setup an sftp server".