noblues,
Some answers and info that may help you from an ltma north valid person.
It is likely you were stopped off at 6000 ft due to your impending proximity to inbound traffic off either BNN or LAM depending on which end you had just departed.
Frstly, some causes of this , not in any particular order,
1.Poor judgement on our part of your climb rate and position regarding inbounds.
2.Traffic avoiding weather in or around the holds
3.LLdir takes traffic off LAM/BNN last minute when we (tma) expect it to continue in the hold.
4...plus many other scenarios.
As for why you are asked to stop at altitude 6000 rather than a flight level is the fact that the traffic you are probably being stopped off against will be at ' minimum stack level ' which is the first flight level above 6000ft which provides 1000ft minimum vertical separation ( sorry if thats something you already know ).
The tcas issue could be covered by the fact that the traffic you might be being stopped against may be some distance away from you, at a very strange bearing turning away from you, ie you off 27l/r against aircraft at BNN just starting a tight turn to leave BNN on a heading. not a tcas expert, just a thought.
As for why you didnt get a sorry, well the controller has probably just done something rather exciting and is a tad focussed on the outcome and the actions to resolve it and in contact with, if necessary, other controllers whose traffic they have nearly smote a blow with.
Please also be very aware that our radar picture is a few seconds behind reality and that whilst you amy be soaring past 6000ft our radar screen could be showing anything between 5-5700 ft - somebody will provide a more accurate figure involving radar update/real time delay.
I am sure that virtually all of us appreciate the consequences for you of such a late instruction however in my opinion its by far a better option than avoiding action a few miles later.
As for going into paperwork, entirely your decision. Last tip, take the time for a visit to LTCC, 10 minutes drive from LL with lots of easy parking - until the suits arrive from hq.
hope that is a little for you to go on.