How important is it to learn Javascript in order to secure a Google Analytics job and/or progress/succeed in such a role for the long-term?

There are multiple paths you can take in analytics. An analyst really can get by for a long time without writing their own JavaScript. You will occasionally need to send code samples from Google's site to developers to get better tagging implemented, but not write from scratch. This describes 95% of people who work in analytics by my estimation.

If you want to innovate in analytics/be a technical specialist and work on the hard jobs, then JavaScript is absolutely necessary. You can't do that job without those skills. It can be more lucrative to work here, but there is usually less of a need because there are already a lot of people who can do this and you usually only need one per organization.

So I'd say you're fine without learning it, unless you want to be a highly technical consultant who innovates/solves the hardest problems vs. run of the mill stuff.