I think this issue actually solved itself - or it was a caching issue (I had purged the cache immediately post upgrade, but have since done a deeper cache flush from the cPanel interface for the SiteGround Optimizer plugin) which sorted out the preview.css issue.
However, there then popped up a new issue to do with js files. The builder was loading only the header and footer, and the main content area was not showing up at all. In the chrome console, this was the error showing up:
cornerstone-site-body.min.js:1 Uncaught SyntaxError: Malformed arrow function parameter list
x-site.min.js:1 Uncaught SyntaxError: Malformed arrow function parameter list
react-dom.min.js:103 TypeError: Cannot read property 'everinit' of undefined
react-dom.min.js:53 Uncaught TypeError: Cannot read property 'everinit' of undefined
Which was then resolved by turning off just the JS Minification in the cache plugin being used (SiteGround Optimizer). I was able to turn back on JS Minification, with specific file exclusion for /themes/pro/cornerstone/assets/dist/js/site/cs.847fab8.js and /themes/pro/framework/dist/js/site/x.js, and everything worked as expected.
I guess this is not a massive issue, as JS Minification is often something that requires a bit of care when turning on, however it was never a problem with previous version of Pro (currently all sites I’ve built have the SG Optimizer, and most have JS Minification on with no issue—front-end of back-end) so I wonder if it is something that can be sorted out?