I think it's more about the level of interactivity that is "needed". The seller part of the platform would do a lot of customization and changing settings, some of those probably have live previews as well, whereas the seller part of the platform is pretty static, no need for the same level of interactivity, so doing it by manually manipulating the DOM is manageable.

server side rendering also points you toward more cacheable content. Things you put on a CDN are faster and way cheaper.

In theory it shouldn't matter but you can SSR with React if that's important.

Problem is SSR with a JS framework is often slower than the other equivalents, not sure why but I see it time and again

Then the hydrations costs on the client typically start at 500ms, and that's 500ms the main thread could be doing something useful with