Links between individual product detail pages

Site URL: https://www.antoineart.com

I would like to add links from some of my product detail pages to other related ones. For instance, I would like to add a link from this original painting's product page  to the product detail page that list it's prints

I understand that I can add a link using the second page's URL. I would prefer linking to the page directly. I'm able to link to product landing pages but not the product pages themselves. I prefer not to link using the URL  -should I change that in the future my link would of course not work.