I have managed to easily install the Matomo web analytics software (basically following these instructions: https://matomo.org/docs/installation/).

Some Opalstack-specific pointers in case someone else wants to try it:

I created a new subdomain ("analytics.mysite.com")
I created a new application using the PHP-FPM+Nginx application type, not changing the defaults ("analytics-mysite-com-php")
I created a site route ("analytics-mysite-com") to link the subdomain and application. I enabled LetsEncrypt and set the "analytics-mysite-com-php" application to serve "/"
Later, once the certificate had been created, I went back and turned on the "Redirect to HTTPS"
I created a new MariaDB database ("username_matomo")
I got hold of matomo from the shell on my Opalstack server as it suggests (wget https://builds.matomo.org/matomo.zip).
I unzipped that and then moved the files contained in the new matomo folder into the app folder (e.g. mv matomo/* ~/apps/analytics-mysite-com-php)

Other than that, it was a case of following the Matomo instructions - very easy really.

I have found two issues relating to Matomo:

Some features of Matomo require access to PHP's shell_exec function. I am not sure what actually. The basics seem just fine. I have found that the GoogleAnalyticsImporter does not work without it. Is there any chance it could be enabled? I guess not.
The Matomo system check also says "It is recommended to configure a 'max_allowed_packet' size in your MySQL database of at least 64MB. Configured is currently 16MB." Again, I don't actually know what the consequence of having a smaller than recommended packet size is. Can that be increased?

I found two issues when creating the database:

When creating a new database the explanation for the "Name" field is "The name used to request a MariaDB database and a user assigned to access it.". I was quite confused, thinking that I had to somehow put both in the field. I think it means that the database and the database user are called the same.
If you type in a name that has already been taken and press "Add Database", the request goes off and I can see in the browser's dev tools a response come back from the server with an error in it ({"errors": ["Database user with that name exists."]}) but the web page doesn't react to the error so it looks like nothing has happened.