Is getting called to the bar important?
Yes, getting called to the bar is important. Go on a job search website and look up in-house positions. Almost all will require you to have been called to the bar.
Sure if you start working now you can earn 3-4k a month, but your progression will be slow and it will honestly be extremely difficult to negotiate an increase in salary thereafter.
Public Sector Legal Roles
Getting a legal role without any experience (even in public sector) is difficult and to be honest, I have yet to see a legal role in public sector that doesn't require you to have been called to the bar. What they do offer, though, is training for you to get called to the bar - this option is great because they tend to pay you full pay (you will be a legal officer and be able to rep the org in court). These job Ads will usually include a line which informs you that working exp with them counts towards the training requirements under the LPA. I've seen one for
MOM in the past... but you'll still have to do part B.
I'm not going to be retained!
Worry less about retention rates and more about your abilities as a lawyer. Gain as much exposure as you can and learn as much as you can. My CV always had a list of cases i've assisted on and the contributions i've made in those cases. Its about how much you've done and how well you can market yourself.
I would even say that you're in a better position because you know for sure you won't be retained and can start looking for full time opportunities almost immediately after you start (well... give it 3 months). This means you won't be in a situation where your job search is dependent on when your employer decided to tell you if you will be retained.
Finally
Make a commitment and stick to it. You've already spent 4 years in law school. At this point, what is that one additional year?