As far as the UPS route, Quite a few people have done this in the past, and there are a number of existing threads and project reports on it.
Here’s the FAQ on planning for outages. (The topic title is a clickable link). I would start with that because it’s summarizes a lot of the issues you are likely to run into that you may not be aware of.
After that, you can take a look at some of the existing project reports on the following list:
https://community.smartthings.com/tag/project_power
Just remember that smartthings is primarily a cloud-based system. Providing back ups for power and internet connectivity are helpful for some people, but if the smartthings cloud is out, or if there’s an undocumented platform change that causes some of your stuff to stop working, there just isn’t anything you can do about that. 
In fact, there is very likely to be at least one or two very small outages every single month, just because historically there always has been. Smartthings issues hub updates that individual customers can neither defer or deny, and they will take your hub off-line. Sometimes only for a minute or two, but sometimes for considerably longer.
But again, read the FAQ, it discusses the various points of failure, and then you can start to get a sense of whether it’s worth it to you to invest a lot of money and time and creativity into improving, say, three of five weak points when you know there’s nothing you can do about the other two 