Prefer 5GHz over 2.4Ghz without disabling 2.4Ghz
You can take advantage of  the experimental "prefer 5 GHz" feature from WiFi band – or Smart WiFi Selector, which offers that as well:

 
WiFi Band / Smart WiFi Selector (source: Google Play; click images for larger variants)

Unfortunately, WiFi Band comes with AdMob and Firebase, thus not fully meeting the requirements.



Disclaimer ahead! 

I haven't really tested all these applications, so I'm not sure if they fall into the acceptable criteria (firebase, google dependencies etc) besides my device comes with this feature already: 

Acknowledgements: How to change from 5GHZ to 2.5GHZ frequency on Android N



In terms of potential 3rd party apps, the closest I have come across is WIFi 5, which is dated , but offers such functionality. It specifies the signal level to switch to a 5Ghz access point and displays a notification showing the available bands.



WiFi 5 was last update 2012, unlisted from Playstore in 12/2017, and thus is only available via ApkPure currently. Not sure whether to trust that site.