@rebekah_creates Oh ... so many things!
If G see's big changes, it has to re-evaluate :(
Plus...
1) Change of mark-up (loss of headings etc.)
2) Spammy mark-up (using headings in links etc.)
3) Change to Nav organisation/wording
4) Change/Loss of internal sub-navs (breadcrumbs, related etc.)

@rebekah_creates 5) Complexity (more nesting, separation between blocks of content etc.)
6) JS Reliance (is the vital stuff in plain HTML, and is it "visible" to users without JS?)
7) JS User Interaction Reliance (content may be loaded with JS ... but does it rely on clicks/scroll etc. first?)

@rebekah_creates 8) Speed performance ... is it slower, and how much, and what (TTFB, DOM Complete, First Paint, First Contentful Paint etc.)?
9) Render/Display issues ... CLS/LCP, as they worse than before? If so, by how much?
10) URLs ... not SPA?
Redesigns can be an absolute minefield

@rebekah_creates Sadly, many themes (and custom "designs") tend to lack in UX and Speed ... things like lacking lazy-load, prioritised/optimised load order etc.
And some themes don't come with the same sub-content blocks (so you lose some internal links etc.)
The only way to be sure ...

@rebekah_creates ... is to roll it back and wait a few weeks.
If it bounces back - roll it forward and see if it drops again.
(I know - that stinks - but it's testing/confirming ... and may be sufficient to prevent a sucky design crippling a business)