notNot exactly a veteran myself but iI have done a number of extract brews with a partial boil as you describe. latest
Latest guidelines for such partial boils dontdon't typical call for pre-sanitizing the top-off water. modern Modern tap water is apparently "clean enough". the The pre-boiled method you describe is the only alternative iI am aware of. the The other is to add all the top off water just after end of boil, the mixing with the near boiling hot partial boil wort apparently does the sanitizing for you. iI usually cool first, then top-off. as
As for when to add LME, it depends on the recipe. mostMost extracts iI have done call for LME added at start of boil, with some occasionally adding more LME near then end as you call out. hope
Hope this helps.