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Just to follow up - I have now actually 410'd the pages and the 410's are still being re-indexed.

My company currently uses an EU subdomain (similar approach to what you're describing, except varying the subdomain rather than the TLD). But one of the issues we have found is that the search engines have no geo-targeting options for Europe as a whole. They allow to target a single country. Or a single language. Or a country-language combination. But there are no options to target "Europe". So, with hreflang tags, and also with GSC settings, you will experience some challenges because of this. We haven't resolved all our issues, whcih is why I'm not providing a solution to you, but we are far enough in to know it is problematic.

Perfect! Happy to help!

All of the plugins I can find allow the tag to be deployed on pages, posts etc. You pick from a pre-defined list of existing content, instead of just whacking in a URL and having it inserted (annoying!) If you put an index.php at that location (the location of the 404), you could put whatever you wanted in it. Might work (maybe test with one). Would resolve a 200 so you'd then need to force a 410 over the top. Not very scalable though...

Thank you for the help, Gaston!

Using the htaccess I 404'd all the pages using "Options All -Indexes". Will this resolve the issue?

Thanks for the help, Nicholas!

No. Just keep an eye on it. If you continue to see impressions and clicks, scan your site again just to make sure all content has been converted and there aren't any lingering files hardcoded with http. You server-side redirect will take care of it. The only possible downside is the unnecessary redirect which slows rendering a bit. Not a show stopper.

you guys are awesome! Thank you.