If you are still going on the path of renouncing your PR and reapplying under the express entry program:
If your Canadian experience was within the past 3 years you can claim points under CEC.
Alternatively, you can also try for the PNP programs where you can get higher points if nominated by the province.
One more option on a long route is applying for a study permit as a post grad or PHD program and working on that to add to your points for Canadian education as well.
Tax filing doesn't quite help - but it keeps your previous resident profile legit.