You are right, the footer container is floating and has always the same distance to the item and total sum container. So, If you have an invoice with just one line than yes, the footer text is quite high. But the footer text will move down with every line in your invoice.
You can move it down. Just go to Account Settings>>Invoice customisation and click footer. Click inside the footer text box(on the right handside) above your footer text and enter empty lines by pressing just enter. Your footer text will move down. But it only works when all your invoices have the same amount of lines/items. If you have an invoice with more than your usual lines the footer will move even more down, maybe on the next page. Also keep in mind that there is the pdf footer container below the floating footer. That means you will not get the footer right to the bottom of the page.
I think that is not the best method and it is not really working.
If you use the Fixed PDF Footer Text, as Matthew mentioned, that is only shown on PDF invoices and always sticks at the bottom line.