Obtaining abstracts for symposium talks before vs after submitting proposal
Regarding your question:


  How is it best to time the symposium proposal's submission relative to obtaining the full talk details (titles &amp; abstracts) from speakers? 


This is specific to each conference. Check submission requirements. It is likely that they will submit abstracts to the conference and your session after your session is listed on the website/conference registration. Additionally, it is likely that what someone proposes to present now (early) may be different from what they might propose in three months when they register or when you, as a group, chat about the content of the session.

I, too, am an ECR, so consider that when reading my answer.

A few years ago I wanted to propose a symposium on a topic at a 'major' conference. I reached out to this professor, whom I had only brief email communication with, to gauge their interest in the topic. I provided a list of potential speakers and an abstract to this person. After some discussion, we concluded that a different, but related topic might be better for the conference. We submitted and it was accepted -- it was great! We recently published a paper which resulted from that meeting/symposium.

This leads me to your question:


  Is there a real career benefit in having 'symposium organiser at
  conference X' in one's CV, i.e. are there real points 'scored' with
  this activity when applying for grants &amp; jobs?


Yes! You will grow your network. You will learn through the activity how to strategically propose sessions, and invite and collaborate with others. You should come out of the session with a new, smaller network of researchers with whom collaboration should be a little easier (now that you 'know' each other). If the society accepts your symposium, it is likely they would be interested in receiving a research article on the topic(s). Organizing a session is also great for working with people with whom you likely wouldn't physically meet otherwise, and for getting out of your daily working area (i.e., invite people from regions/groups different than yours!).

Unless you are making travel arrangements and securing funding for the invited speakers, I don't think there will be much leg work beyond the initial submission, invitation emails, and reminder emails. 

Find a mentor and collaborator to guide you through this process, and to help build your network.