The four essentials for a fast 2k that spring instantly to mind are:
Perfect technique;
Vast amounts just below AT (80-90% of total);
Vast amounts above AT (balance);
Pacing.
Clearly there may be others and equally clearly you do the ones you haven't already done. They are all
sine qua non, but technique makes you do the others even better. For moving boats fast, there's nothing like putting your blade in quick, well forward of the gate, and keeping it covered and pulling til well astern.
If you want a schedule, try the Wolverine L4 or one of these:
http://concept2.co.uk/training/interactive
The variety is itself important, we learn what happens under all circumstances and how to use it if needed.
The single best eye-opener however, is perhaps the 30' rate controlled piece, either fixed rate such as 30'/20 with the last 10' as hard as possible, or L4 style 18-20-22-20-18/2' each etc. If you're not M25+/-7 and 6'+ you can adjust the ratings slightly, according.