discussion with a CAME (Civil Aviation Medical Examiner) who advised that the Canadian civil aviation medical process typically takes around six months
I may be a year out of date since I recently returned from an office job to line flying, but Transport always seemed to prioritize those working over those applying. For example, an FO I worked with at my last company had his ATPL in hand after only about three weeks because we went to bat for him saying his upgrade would be delayed. But a new person, off the street - good luck getting that same level of service. I'd agree with your CAME - expect at least six months for the medical, and just as long again for the license.
Given this timeline, I'm curious to know if now is the right time to start preparing for the SAMRA, SARON, and INRAT exams
Start studying now. Transport Canada exams are as much an exercise in English comprehension as they are about aviation knowledge, with some questions featuring a "double negative" that can only be answered if you fully understand the topic. While no one cares about your exam results once you see "pass," a lower pass score will eventually cause you issues when you transition into larger and faster aircraft. When teaching on my current machine, I get to assume you fully understand all the required knowledge about, say, Mach Tuck, or how to avoid turbulence around a jet stream, or how a hydraulic or electric system works. I'm not going to spend even 3 minutes trying to explain it to you. That's your responsibility as a professional. Leaving this sort of thing to the last minute will only cause you stress down the road.
I would suggest that if you can get your hands on an aircraft manual - King Air, Dash 8, ATR - you'll find some of the material easier to understand. I say that because if you understand how a system works on an actual airplane, then the examples they give during the exam become a variation of a theme rather than some lofty idea. Take what a book or course says and then compare that to how it works on that aircraft. Ideally, the aircraft you pick will be a complex turbo-prop or jet (ideally both as I had both prop and jet questions on my exams).
Is Nizus subscription sufficient for exams preparation ?
There is no requirement for you to front any money, except for the textbooks. While these programs are great at getting you a pass, they're not all so great in having you understand the material enough to operate with because the "why" is often lost to pretty pictures or animations. I'm not saying don't do it, I'm saying that no, a subscription to an online course is not sufficient. Plus, you likely clicked a box that said they are not responsible for missing content or out-of-date information, so always double-check with the source material, especially for subjects such as Air Law.