Filing taxes in Canada is relatively simple compared to the USA. See the site for various options, but try to find ones not operated by American firms such as H&R Block and TurboTax.
From a cursory search, TaxTron, GenuTax, Better Tax appear Canadian. Many are Pay-What-You-Want model.
If you have a complex tax situation and/or can’t file online, and/or don’t want to do the paper return yourself, consider seeing an independent Canadian accountant.
I could not find their API documentation, so I imagine the current NETFILE might not be too far from that.
As someone who has filed via NETFILE a number of years, you fill it out by hand as much as you write Word docs by hand, or less. Tax software collects the information, you push a button, it asks for your CRA credentials, and sends the properly formatted file it prepares. I imagine that, even if you were writing a program to use this, there are two parts. The first is the file format and the second is the communications requirements.
The hardest part was connecting to CRA directly. If you have that worked out, great. If you want to avoid that pain, link it to your bank account and your bank authenticates you, and then it’s as complicated as logging into your online bank app/site.