Hi, here I address some of your main concerns.

I need POST service calls matching the path of /service/* to be directed to endpoints hosted by another service on port 80 (the website is hosting on port 8888).

SWS only supports GET, HEAD, and OPTIONS request methods. That's why you will get 405 Method Not Allowed. See https://static-web-server.net/features/http-methods/

so the rewrites seem to work as long as it rewrites to a URL within the static-web-server, but as soon as change the destination to something external it doesnt work.

URL Rewrites supports a redirect option to tell SWS to perform a redirection request when the source matches. You could use that option.
However, take into your account that SWS is not a proxy, so if you use an external URL than a local path then you probably want to enable the redirect option (which is there in Rewrites only for convenience) or convert your Rewrite rule to be a Redirect rule instead.

URL Rewrites are for local path usage mainly and as I said before, SWS is not a proxy server therefore if you want to bridge requests from SWS to elsewhere then SWS is not for you.