Just to be clear, eTA is what people get to visit Canada if on a non visa requirement passport. So basically most so called “1st world countries”. Excluding USA.
Having a work permit doesn’t exempt you from getting and needing an eTA. The only exceptions to eTA if from a country that requires one are CoPR, PR card, or obviously you don’t need one on Canadian/US passport. If you are on say a Indian passport, you need a visa, not an eTA. There may be an exemption for say US PR card holders, diplomatic passports etc but I’m covering the basics.
Now all that aside ... If you are entering Canada to work, even if it’s not for a Canadian company except to get interrogated and potentially rejected and be extremely clear what you are doing lest you get barred for a year or more, subject to secondary every visit and you’d need to declare it on any future application.
I don’t know enough about your plans nor am I am expert on the various options that don’t require a permit... I suggest you contact CIC directly (could be time consuming) or hire a lawyer / immigration specialist. I have a couple of experiences with companies that I will happily share over PM if you want.
Theres also a few good immigration sites out there, but to be honest without full details of your plan no one can tell you for sure. And I am not and wouldn’t suggest posting them here, I think you are better off going direct with an expert here; people say the only two certainties in life are death and taxes. CIC is a 3rd.