I'm looking for the functionality to set certain products within a store to 'offline / ins-store' purchase only.
So pricing is displayed alongside all usual product details but the 'add to cart' button changes to 'In Store Only'. I'm surprised Shopify doesn't have this built in natively as it must be a fairly common requirements for a lot of businesses.
You can simply achieve this with custom code.
If you want it only in product single page then create an alternate template for product and remove/comment 'add to cart feature' and append your desired functionality.
If want it in loop like collection or homepage grids then use specific tag and modify code accordingly.