Ask what the first byte must contain
If search engines, link previews and first-paint speed matter, rendering on the server is not optional. If the product sits behind a login and users stay for an hour, a client-rendered application is perfectly reasonable.
Frameworks buy conventions
Routing, data loading and build configuration are solved problems. A framework's real value is that a new engineer recognises the structure on day one.
Mixing is normal
A server-rendered marketing site alongside a client-heavy application is a common and healthy split, as long as they share a design system.