Really depends on what you want from the voice notification. For example, are you just looking for notifications that will be mostly static like "Front door is open", "Back door is open"? If so, then you can buy Echo dots for less, especially right now at just $30 each. Mute their microphones if you're concerned about being monitored.
Use this method for triggering them. No need to own or have ever owned a SmartThings hub. Ignore the part about Google Assistant Relay, and instead trigger the SmartThings switches with IFTTT. When those virtual switches are triggered on SmartThings, it will trigger your Alexa routine to speak your custom phrase. You'll have to create one IFTTT Applet, one virtual switch on SmartThings, and one Alexa routine for each custom phrase, but once you do it a few times, it becomes simple and you'll be able to setup additional custom announcements in a few minutes.
The one drawback to using Echo at this time (if you consider it a drawback), is you have to create a routine for each of your echo dots. You can use the same IFTTT trigger and the same SmartThings virtual switch for a custom phrase you want spoken amongst all of your echo dots at once, but the Alexa app doesn't allow you to execute a single routine on multiple Echos at once. However, you can send the same message via multiple routines, to multiple Echos at once. For example, when my front door triggers, it simultaneously triggers two Alexa routines, to my two Echos at the same time and announces "Front Door". Some people consider this an asset, since you can send some announcements to all your Echos and some announcements to only certain Echos.
