i wouldn’t be overly concerned about that Diagnostic. It becomes an issue when the DOM has over a total of 1500 nodes, a greater Depth of 32 nodes and a parent element has over 60 child nodes.
However to answer your question. The case for inline SVGs:
1. They are contextual ie. a button icon or a prefix to a headline.
2. They have a small number of child nodes e.g paths. I rarely use SVGs with more that 15 paths. So maybe a ceiling of 30 would be a good cap.
I think at least one of these rules should apply before using them.
The main culprit on your site is the WP_Maintenance_Security icon which is being flagged as having 56 child elements. Considering that it is neither a button or a supplement to a headline but a standalone image, i would add this as an <img>