Well...
Again, lazy load is a very good idea, especially for "heavy" pages (lots of images) and for bandwidth-limited audiences... Provided it is not implemented in such a way that it's more trouble than it's worth. It is entirely possible to implement lazy load without getting a bad score on various page tests.
But it's hard for me to comment technically on a page I can't explore in practice.
However - I don't think you need a technical vocabulary to talk to your developer.
It looks to me like you've come to a situation that happens A LOT in this area: the customer gives the developer a paid job and the developer thinks up ways of making things really sophisticated in order to have an argument for the billing. However, this achieves the opposite result when the customer considers the product too elaborate and is less happy than before the task started.
You mentioned Drupal and Wordpress - the trouble with these is that there are a gazillion of pre-packaged solutions which are general in nature and usually not really optimized for somebody's particular situation. And it's too easy to just grab some of these, plop them together and hope the client is impressed by the visuals. (That's a general comment - again, I can't talk in detail about a page I haven't seen in practice.)
Just let your developer know that you want to keep things simple - no complex loading schemes, no special effects, no eye candy (that is technically "heavy", increasing page size and loading time), no massive page length. Anybody can understand that - if they are ready to understand it.
Now comes the problem part.
Your goal is entirely legitimate and a good idea.
But from the developer's point of view what you want is something boringly simple that doesn't really put bread on the table for them. It could happen that it becomes hard to find a person that will be willing to do it, maybe a web-literate student. In my
experience I've often told a customer "look, you want X, I know what this is really worth and I know what my minimum price is to dedicate time to a
project - we'll both be happier if we don't to this together".