Turan, sorry you have having such grief.

A <defs> block is coding something for later and repeated use.
It would be accessed with a <use> block.

I can see Xara is including the <defs> to add in a Google Font, in my case, Opens Sans.
boxy-svg.com can define complex filters with and id and then reference the id in a url.
If no <use> block or url reference then you can probably remove the <defs>.

I use the svgomg website directly.

For the Xara SVG generator 6.6.0.1, I get no faults.
I sometimes get the DTD complaint in 6.6.0.6 but the designs still render.
I have exported Adobe, Inkscape and non-repeating and frankly here is no difference in the SVG code.

AI uses a Comment:
<!-- Generator: Adobe Illustrator 25.0.0, SVG Export Plug-In . SVG Version: 6.00 Build 0) -->
Xara does the same:
<!--Generator: Xara Designer (www.xara.com), SVG filter version: 6.6.0.6-->
zint.org.uk doesn't have one.

I doubt then that here is a Xara SVG Export fault as I cannot get any load issues in any of the tools I have.

Acorn