For every level on rows (4) or columns (2) you can specify more than one dimension type, you are not limited to the selected one. But only the selected dimension type is dynamic and can be used with list functions like .base or .tree. As a work around, you can create a xfbr function that creates comma separated lists of concatenated members in a scripts and with this, you can extend the row level limit beyond 4. With the Header overrides, you can show these members in the header independent from the selected dimension types in the rows or columns. If you need more than 6, you can works with name().