No raspberry pi is needed with Echo if you just want custom announcements. 
You can make any Echo device speak any custom phrase (or play from a variety of sound effects, including sirens, doorbell chimes, or dogs barking) by using an Alexa routine (not a smartthings routine).
https://www.amazon.com/gp/help/customer/display.html?nodeId=202200080
You can integrate that with smartthings by using a virtual sensor to trigger the Alexa routine. This is a very popular use case. See the community FAQ.
FAQ: Can I trigger an Echo Action without Speaking to It?
No raspberry pi or any other device is required and it’s using all standard features except for the virtual switch.
It’s not as flexible as the methods which require a raspberry pi because you will have to set up a separate Alexa routine for each phrase that you want spoken, but it’s definitely doable.
Unfortunately there is not as yet a similar method for Google home.