Just using the permalink settings feature on WordPress dashboard to make the WordPress links look exactly like as it was on Squarespace. Of course, you will copy all the pages over.
Ensure the URLs look exactly as they were on Squarespace. Failure to do that would result in the death of the site. Any page that you can get the URL exactly, create an appropriate page and redirect the URL.
Its a massive site. The new algo prefers reducing useless pages. Would you redirect useless pages or copy them anyways.
You don't want the useless pages preserved, you don't want bad quality pages index anyway, plus you want good to focus on the other pages. If the urls are the same for the pages you want great, pruning useless pages is good for SEO anyway even if you weren't migrating, just like trimming a weed plant while it's growing of it's excess leaves more of the plants energy is directed to the buds(the useful pages with preserved URLs), same thing basically.
Redirect them. If they're useless, why keep them?
I often use deep links to SEO BHW. If your old links are not effective. Let's do another method
remove any page with no purpose or traffic, basically made to just add content, adding 410 can be a good idea here as well,