At present we have all our products as simple products, but some of these are similar with different sizes etc, so should really be set up as a configurable product. We want to change these so, all skus still show in the product category, but navigating into one of those pages includes the different options. When clicking a different option, it reloads the page with the unique URL from that sku. Example https://www.ikea.com/gb/en/p/malm-chest-of-3-drawers-grey-stained-10482512/ has easy options that use a unique URL. Amazon also do it like this. In magento it seems you need to create a new sku for a configurable product, and it doesn’t seem like you can select existing skus.
https://www.axminstertools.com/nitrotouch-glove-size-8-m-953639 is a M2 website, that is fast loading when changing the product options with unique URLs.