Miklos - You can define any frequently-used combination/grouping of filter settings as Named Queries, which users can then access by name directly from the Sidebar. Defined Named Queries in your file are also available on the Subset drop down of each View Toolbar. You can use your list of categories Sidebar filter just once to define most of the combinations from your Categorised list of sites, e.g. only sites A and D become a Named Query called "A&D". Be sure to do a global reset before defining each new Named Query on the Queries drop-down using 'Create query from filters'.

There are more advanced options available for even larger lists of text using the Text Tools drop down on text column/field Sidebar filters. For example, if your site list has predicable patterns in the text, you can define text filters based on the first 'n' characters, then change the text filter Text Tools setting drop down to "Word prefix" and all rows with site names starting with the same text will become part of the named Query encapsulating this text filter setting. Another Text Tools drop-down option for filtering Text is to change the filter option for a text field to "Regular Expression", which are used to find repeated patterns in text strings.

Another idea is to use selections in the table view and to add them to a named query.

Or, use a narrow Bar view configured to split by sites with 100% bars. You can multi-select separate regions in the bar view.