Q1 - are you using meta data, words numbers etc as custom dimensions to analyze SEO? Do you do it for real? isn't it an issue in terms of database size? like firing every time the page is loaded (author name, word counts...)
Q2 - do you have any solutions when it comes to measure the number of backlinks pointing to a specific domain, in order to store this value within Matomo?
Q5 - Have you ever heard of interest on the side of SEM Rush to include competitors data within Matomo as a plugin? Typically a simple widget plugin like Amperage did with Crazy Egg and Moz?
Q8 - How do you manage the catch all titles of a given page? Are the data sent as a list of values or is it through the custom variables which can be multivaluate?
I've personally been playing with custom dimension for "Average reading time", on a small site.
But we have clients with massive amounts of data using similar techniques. Have not heard of any issues yet.