The solution to the assets not being available/found is:

1. With Rails 5 and later (I believe?), the syntax of the module spec changes: Instead of jquery.ui it becomes jquery-ui. This applies to the stylesheets and javascripts.
2. Once the above hurdle was crossed, I also needed the ExecJS gem and install nodejs on the host. Since this was a new installation, that had not been done.

Thankyou Alex for the pointers and tips as well.

The application now runs on with Rails 8!