Re-ranking a website after replacing URLs and deleting pages

I have a special case for today for all SEO specialists and I hope to get your opinion on the best solution.
I am working to rank an ecom website that has too many problems from SEO point of view.
The site was built through code (actually a mix of 3 programming languages) and one of the serious problems was that URLs were not SEO friendly and no reflangs. (but the website has a blog which is wordpress) The website is in two languages, English and Greek and now the developers set the algorithm to read the English title from product pages and created the URL and inserted it manually for the Greek URLs. Moreover the site will move from http to https in the same time with the changes of URLs.
So the old URLs will be totally replaced (but not with re-directs) by new URLs.
If you ask me why not redirects I have to tell you that we are talking about thousands of pages.
I managed to rank the website only with the home page and some blog articles (the ones that I re-wrote the content since almost all blog posts were double-content). The English version of the product pages is double content but the Greek are fine.
The question is: I have a few hundreds of pages which are for old products that will never be again in stock. It looks to me that at this point since most probably the ranking will start from 0 again with the changes of the new URLs the best solution is to delete the unnecessary pages and replace on the social media all the links with the new links and delete the posts with the old products so we limit the chances of broken links (BTW the site doesn't have any back links so the only links available through the web are the links on the facebook page)
BTW we don't have the option of un-publishing pages ( It is something that we are still discussing with the developer) however I think that having in mind that they will be hundreds at this point it will be an unnecessary weight for the site...which most probably will slow the speed....
Any experience from someone with such cases? To be honest it's my first time that I am thinking to delete pages..until now I was fighting for each link not to have a broken link..funny heh...