For the major updates in Google algorithm like the recent one for medical websites, Is it better to postpone the website migration to after update launch or to speed it up to do the migration before update launch?
Migrate when you need to - don't worry about the Google. Google will change, tweak, introduce new algorithms...
Worry about your website's contents, make sure it's easy to navigate for the visitors. Of course, us all the appropriate tags, so that search engines can also navigate it more easily and "figure out" what's what, but that's where it ends.
Trying to "outsmart" Google, in whichever way imagineable is not a good long term strategy, nor a cost/time effective one.
As for good SEO tagging, alt descriptions, directory structure, file names etc - there's loads of info on google's own pages and on numerous other SEO related websites.