While not a fan of SAFe or any agile cargo cult implementation there were a few things I liked about SAFe: Some acknowledgement of the role of planning which imho. is needed at that scale and some framework to handle architectural concerns.
Or have a look at Coplien's Scrum patterns [1] patterns that do involve "planning", and that are also supplemented by his decade old organizational pattern work outside of just Scrum.
[0] https://www.holacracy.org/constitution/5
[1] http://scrumbook.org/ and https://sites.google.com/a/scrumplop.org/published-patterns/...
If you can do that, you can do agile. It doesn't matter if your customers are internal or external - internal might be better, because you can work with them more regularly.