From my experience, Java while more verbose offers better tooling, standards, documentation and support. Scala certainly is more concise but there is a cost that you will incur later when trying to debug or maintain s large fast moving code base. Sbt blows, ide support for scala is spotty and good luck defining a code convention and standard that you can automatically lint and format. To me it's c++ for the jvm (not a good thing ☺).
The next thing I'd advise is to not use play but to consider dropwizard or better yet spring boot. You may be thinking "ugh spring.. Isn't that so heavy?" Possibly but it's a lot simpler to use, extremely powerful and well documented and supported. Play just doesn't have that maturity and support going for it that a spring or dropwizard have.
totally! I think the analogy of going back to school is a great one and is the same rationale that I used when I left my job. 10+ years is a good long time so you could at least enjoy the benefits for a while before joining your startup :-)
Moving away from Rails need not be the solution. You can build a service based architecture purely in Rails if you want just as long as you have clear contracts between components (have a bunch of services still built using Rails). Many times your "monorail" app has a bunch of tools and other components glommed together all sharing the same ActiveRecord models. Find those split points and put proper data layers around the various points and build clean contracts.