New site on an old domain: how to minimize organic traffic drawdown?

Question

Search Engine Optimization

Good afternoon.
There is a site of one business related to services, made on a wix-type template (up to 10 pages). The business is redeemed - access to the site is included in the price. The old site, although outdated and lacking in terms of design and usability, is well advanced in a number of queries in the top 5 and top 10.
Question 1: what are the ways to minimize organic drawdown when creating a new site on the same domain?
Question 2: a familiar marketer voiced the idea that you need to save all the pages of the old site along with the URLs and transfer these URLs in the form of links to the appropriate sections of the new site, you can in the most invisible place. At the same time, the future proger expressed the opposite idea that usually in this case, SEO customers ask the urls to leave the old ones and sculpt new content on them. I am confused.
Thanks in advance.

Answer 1

It is better to leave the old urls and content.
However, if the URLs are terrible, then use 301 redirects to transfer pages to new URLs, and leave the old content. The drawdown during regluing, if any, is small, then everything will return to its place.

Answer 2

It is better if the old urls with the same content remain working.
At the very least, you can create new pages, transfer content to them, then catch the url of pages already promoted to the top and do a 301 get rekt redirect from the old uri to the new ones.