I like using HAVING just to have conditions that reference expressions from the SELECT columns.

e.g. rather than having to do

    SELECT
        COALESCE(extract_district(rm.district), extract_district(p.project_name), NULLIF(rm.district, '')) AS district,
        ...
    FROM ...
    WHERE COALESCE(extract_district(rm.district), extract_district(p.project_name), NULLIF(rm.district, '')) IS NOT NULL

just do

    SELECT
        COALESCE(extract_district(rm.district), extract_district(p.project_name), NULLIF(rm.district, '')) AS district,
        ...
    FROM ...
    HAVING district IS NOT NULL

Hopefully the optimizer understands that these are equivalent, I haven't checked.