Good example why using HTMX is usually a great idea. You can do your joins right there in the DB, it's easy and natural.
SPAs are only a good idea if you ignore all the complexity required to make them actually work decently - like carrying over the whole data model all the way to the frontend app.
This is also why SSR JS frameworks are gaining traction. But I still think as a product grows eventually you need some kind of formal API to help separate concerns and scale teams.
Even back in the multi-page application days people doing large apps were already splitting their code MVC style with a model-layer that functions much like an API layer does today in an SPA.
Most devs don't understand that some techniques just don't work after a certain point and others are too complex for simple use cases. Everybody wants their silver bullet...
there's nothing wrong with the browser doing work for the rendering, the issue is that _all_ of the work has been offloaded to the browser.
It used to be, a page that was primarily rendered on the server but used javascript for dynamicism in the frontend was still considered SSR. nowadays people try to relegate SSR to only rendering on the server, but that's never been the case and is a strawman.
SPAs are only a good idea if you ignore all the complexity required to make them actually work decently - like carrying over the whole data model all the way to the frontend app.