I want to create a service that can be used through browser and smartphone applications.

"I want to be RESTful" and "I want to be SSR" already have two properties in the backend.The URL in RESTful represents the resource, and the URL in SSR represents the location of the document.
The main reason I want to SSR is to speed up the initial loading and to deal with Bot and crawlers that don't work well in case of dynamic changes such as OGP.
So SSR has a document in the URL and crawlers are coming.
On the other hand, RESTful is a unique representation of resources in a URL, and there is no element of bot access or document storage.

For example
http://example.com/table/
For the URL
For RESTful, it is the API for the resources in the table. For SSRs, there are documentation in the table.

So
Question 1, Question 2,
As for , I have the impression that it is difficult.
If you have the option of quitting SSR or RESTful, it would be possible.
Or the backend should be completely separated from the site where SSR is performed and the WebAPI where RESTful is performed.

Question 3
I don't have enough information about
Market conditions (share, documentation, support, and development) are important for selecting frameworks and technologies, but the nature of the product (requirements, security levels) and project conditions (budget, people, maintenance) are also important.

For example, even if you choose something with a high share, if you choose something that the project members have not mastered at all, the cost of learning before developing it will be high.
However, if you have to spend your budget thinking about the future and learn in that language, there will be no problem at all.

This is a comprehensive decision, and is an important decision that needs to be made with CTO and other technical leaders and project managers.

If it's for individual learning, you can try what you want to try.
Of course, there are many articles about the difference in annual income and future jobs depending on the language, but you can think about it based on your career plan and what you want to do.
Personally, I think it's more important to put out a product than in which language to make it for learning.If you can learn one language and create a product, you can always learn a new language and make things.