With codementor, it’s designed to give one-on-one help. So the experience will tailored to your needs. But like any one-on-one session, it does have a monetary cost just like a piano lesson.
For Slack, it’s honestly great, but if you are starting out it can feel daunting and not sure what to ask. My advice is don’t be afraid to ask any question even if you THINK it might be “stupid”. The ppl there are pretty nice.
I can’t comment on the Julia course since I haven’t taken them but any course is good IMO.