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Ideally, you'll migrate the entire site to https, and Cyrus' guide is a good one. Google has some helpful info for an http to https migration at https://support.google.com/webmasters/answer/6073543?hl=en.

The canonical tag solution is for the situation where you can't or don't want to go ahead and switch the whole site over to https right away.  Either way, make sure Google knows, either through 301-redirects or canonical tags, that the http and https versions are the same page.

The canonicals appear to be set up correctly, and I would not advise listing the product URLs as their canonicals in the category as suggested above. That will create duplicate URLs with the same content, which is exactly what canonical tags are designed to avoid.

A reduction in the number of pages indexed does not necessarily mean something is wrong. In fact, it could mean that something is right, especially if your rankings are improving.

How are you determining that only 80% of pages are indexed? Can you provide a specific URL that is not being indexed?

If you made changes to your canonical tag, robots.txt , or meta robots tag, these could all cause a reduction in the number of pages being indexed.

If I understand you correctly, you are referring to the following two URLs:

https://www.symectech.com/epos-systems/customer-displays/pole-mounting-kit-94591.html

https://www.symectech.com/pole-mounting-kit-94614.html

Both of these have the same canonical referenced, which is https://www.symectech.com/pole-mounting-kit-94614.html.

It doesn't matter what actually shows in the address box. For the purposes of indexation, what matters is what is referenced in the canonical tag.

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I would be interested in seeing examples of where this has happened. Were the canonical tags added after the URLs were already indexed or were the canonicals in place when the site launched?