There ARE presets you can add to the Alexa app. Go into TuneIn on the app and you can add any station Now Playing to your Favorites list in TuneIn app. I have quite a few added' Just Google for this info and you will find instructions to do so.
No speaking to Alexa required, just tap the station icon you want to play.
When asking Alexa to play a certain station using this example format: "Alexa, play TuneIn station KPLU" works well. If you want to hear last station played (even if after many hours), just say, "Alexa, play last station", and it will play KPLU again.