Look at your landing page report comparing the before and after time period for organic traffic. Is it specific landing pages that have lost traffic or is the 35% drop roughly consistent across the board?
You have a bunch of pages (787) ending in html that are timing out with a 504 Gateway error.
Here are a few examples.
https://www.scottscastles.com/book/enquire/baronial-castle-54.html
https://www.scottscastles.com/book/enquire/beautiful-historic-mansion-31.html
https://www.scottscastles.com/book/enquire/beautiful-holiday-house-23.html
I sampled a few with a site: command. Many are still indexed so that could be the problem. A few are tagged as no-index, but they're the exception rather than the rule. Many are canonicalized to https://www.scottscastles.com/property/enquire/ or https://www.scottscastles.com/property/unavailable/ but none are redirected.
I'd start there.