Why do you think you aren't really buying it? Is it because they allow you to run it without paying money for it?
I don't think the definition of "purchasing" software should be defined by whether you can run the service without paying or not. I think it's best defined as paying money for something that you like and want to exchange value for. In my book that's nothing near a dark pattern, as I can't imagine anyone being confused by it, let alone mistakenly believing there is missing features that they won't get until they buy.
Ownership is being conferred by purchasing immich, that's what the product key codes they've started using indicate.
That is the fundamemental change in the way they are offering Immich: if you pay, you are provided with proof of ownership (product key). If you don't, you are using it as a part of an indefinent trial period.