It's pretty simple, and free, to submit your podcast to TuneIn.
Note: When you are submitting your podcast to some services (like TuneIn) they may tell you that your Acast Open RSS feed is not valid. This is because they are using an out-of-date validation tool (like podba.se or w3c) which does not recognize the new Apple Podcasts iOS tags.
If this happens, please tell them:
My RSS feed is perfectly valid, and conforms to the latest Apple Podcasts iOS standards. Please process my RSS feed as-is.