Hi @stevenh ,
i switched to inertia the moment Laravel 8 came out and I love it. If you know Vue you will work yourself in very quickly as it's basically Vue but without the routing part which is handled through your routes and controllers.
Personally I love the simplicity of having both frontend and backend together without the hassle of having a separate SPA where I basically need to come up with everything from scratch and do the same stuff all over. On top I don't need to take care of any application load balancing or reverse proxying because everything is tied together when it comes to URL structure.
But I do here you regarding the SEO stuff. As far as I understand things, you are not bound to the Inertia stuff. So you can return regular Views on certain routes that could stick to the old fashioned way. It's your controller so it's up to you what you do inside of it.
When you already know that you'll have a mobile app for which you'll need an API then you could try to build things, having this in mind. But if you aren't sure about it, personally I would start just building things and progress as needed.
P.S.: Maybe you give this podcast a shot to find out more about Inertia: https://fullstackradio.com/127