chore(base): Use constructors of SyncTimelineEvent to simplify code

[Hywan]

Extracted from #3585. Just a simple refactoring.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.88%. Comparing base (748f40c) to head (d59b28e).
Report is 54 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3615      +/-   ##
==========================================
+ Coverage   83.86%   83.88%   +0.01%     
==========================================
  Files         254      254              
  Lines       26104    26105       +1     
==========================================
+ Hits        21893    21897       +4     
+ Misses       4211     4208       -3     

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[bnjbvr]

I think we should replace uses of sync_timeline_event!() by using the EventFactory directly, since it usually reads better IMO at call sites, but this works too :)