Google Messenger has become the default messenger app. Once you have it as built-in, not downloaded like would have themes, it will now change the keyboard when dark mode is off. Also, keep in mind that with Samsung Good Lock, you can literally create your own custom keyboard: background photos, key colors, and so much more. Samsung is not doing a good job of explaining that being the default app, as on my S26 Ultra, opens up more abilities than when you download it on the Google Play Store on a non-Samsung phone. As a example you can keep Samsung keyboard The S26 Ultra is the most customizable phone you can imagine; there's literally nothing you can't do. I used Samsung Messaging for 7 years, also thought it was the best, but besides all the latest RCS features and the ability to send full-quality videos and photos, you might be amazed at how much better it is than Samsung Messenger, which has been stagnant for about 4 years now. Also you can change the bubble colors in settings 
As for the keyboard, you can still use Samsung and have all the animations ect. Then use the Samsung Good Lock Keyboard Cafe for limitless designs as well as photos for the background this can be used in Google messenger. As for messenger apps, you can still download anything from the app store if you don't like Google Messenger. Also of side note I have been beta testing for Google messenger and they have lots of new features to come.
