I hope you are not doing all of this alone. I'd try and keep things as simple as possible within a monolith and then improve as needs increase. Good luck :)
I generally write what I consider monolithic Django apps. I would add in Haystack (a search module for Django) and configure it to use Elasticsearch to overcome the problems you describe.
It doesn't sound like microservices are needed, just adding in the appropriate tech for the job.
One hard tech limit is that with 50k podcasts, 4million+ episodes, search definitely doesn't work well. Not just WP, but SQL itself. Hence Elasticsearch. I also plan to work on recommendations, etc. so will need probably to be exporting SQL data into other systems anyway for making the "people who liked this also liked this" kinda things.
Also I kinda lied about using the WP API--that's how I built the system initially (and will switch to it moving forward), but to import the first few million posts from the content of the feeds, I just used wp_insert_post against the DB of new entries that Elixir fetched (I posted the code I used here: http://wordpress.stackexchange.com/a/233786/30906).
I also plan to write the whole front-end in React (including server side rendering) so will have to figure out how to get that done. Would probably use the WP-API with a Node.js app in front of it, will look into hypernova from AirBNB. So probably more usage of WP API accessed by another service...