Do your site contain sitemap.xml? Are these pages really unique? How long it's been since you added them?

There are few reasons for pages not being indexed, we'd need to know the website to research the issue a bit more.

I just think you should give it some time.

You don't really need the 303 redirect.

303 - See other (from RFC 7231) > The 303 (See Other) status code indicates that the server is redirecting the user agent to a different resource, as indicated by a URI in the Location header field, which is intended to provide an indirect response to the original request. A user agent can perform a retrieval request targeting that URI (a GET or HEAD request if using HTTP), which might also be redirected, and present the eventual result as an answer to the original request. Note that the new URI in the Location header field is not considered equivalent to the effective request URI.

Read more about 303 status code

I don't think it's so easy actually. You'd want to keep track of www.example.com even if you don't use it. At least for the data provided - backlinks etc. You can't see that on your non-www version, because it's a different subdomain.

Speaking of https, I'd add https only if my website was completely on https and http only if it was on http. In other cases (when you have http and https sections as well), you should combine.