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Hello SEOhmygod, If you could share your site I would be able to provide better input. From the sound of it though, if I understand correctly, you are canonicalizing each department back to a version of the page for all departments. That is probably not ideal. It would be better to have each department have its own landing page for each category, on which you would customize Title, description and OnPage content for that particular department. You may also want to consider rewriting the URL instead of using a filter so: .com/tyres?department=1 becomes .com/tyres/mountain-biking/ or .com/mountain-biking/tyres

Luckily, 301 redirecting from one site to another (when moving a site) and using the Google Change of Address Tool has taken out a lof of the risk that used to exist. if done properly, you should benefit from combining both sites and not lose any rankings. You're on the right track, I would outline the content and make sure that there's not any content that overlaps--every product/service and every page should be unique and not cause any duplicate content issues. Generally speaking, you'd want to do what you're planning, which is to use a directory off of your main domain and then 301 redirect the pages from the old site to the new pages on the new site. Use the Google Change of Address Tool to tell Google about the change. I would fully integrate the pages into your main navigation on the site rather than just provide a link--it's important that the new pages get all the internal links that they can.

Sean and Logan are right... if you properly redirect http URLs to https URLs on your site, you generally should not lose any search engine rankings as long as you're moving (and redirecting) on the same domain name. A couple of things to watch out for, though, that can cause a site to lose rankings if not done properly: verify all versions of the site in Google Search Console (http://, http://www, https:// and https://www) crawl your own site and change ALL URLs on the site, even references to image files and external files such as .js files, if you're using absolute and not relative URLs. set up only one redirect (multiple redirects can cause issues). Make sure the site doesn't redirect from http:// to http://www and then from http://www to https://www.. make sure your SSL certificate is set up properly (it's easy to screw it up so it gives errors if not set up properly).

Neil, the others are right--you should first show the full content and not hide any of the content on the page like you're doing. Depending on the size of the content, though, you might consider why you're hiding the content in the first place, as you might need to create more pages on your site for that content. Adding the content to new pages on the site might be good for your users, and certainly will fix your problem. When considering the content and indexing, though, if the content is in the page source code then it will be indexed. Google does know if it's hidden, though, as Googlebot, Google's crawler, is essentially a version of Google Chrome.