As I understand it, if you are deemed rehabilitated then you are no longer criminally inadmissible. It depends mostly on whether your convcition is spent, but also partly on what the maximum sentence would have been in Canada. According to Unlock's information, there is a process where you can ask the Canadian High Commission whether you are deemed rehabilitated. If you fill in the application form and tick the box marked "for information only", you won't have to pay the fee but you will still get an answer.
I can't be sure, but I'm guessing that the way these things usually go is that once you're turned down for an eTA, you won't get one, but at some point you might get a visa. So, if they tell you that you are now deemed rehabilitated, then you might still have to apply for a visa.
It has just occurred to me that as you are in Scotland, the Rehabilitation of Offenders Act has been updated/replaced, so your conviction might become spent sooner than it would in England, under the law here, which is the one they specify. That might still be an issue, depending on the details.
Robert Lightfoot, former head of NASA, said it succinctly in his parting speech in April 2018: Protecting against risk and being safe are not the same thing ... [W]e must move from risk management to risk leadership. From a risk management perspective, the safest place to be is on the ground. From a risk leadership perspective, I believe thats the worst place [we] can be.