First things first: do you have a SmartThings hub also, and if so, which model?
Hopefully you do and it’s one that can run edge Drivers, because that’s going to make this all a lot easier, I think.
EDGE DRIVERS
Not sure if @TAustin , Who has been doing some of the most creative integration work with edge drivers, has looked at MQTT integrations or not, yet.
HOME ASSISTANT
Outside of that, most of the people that I know who are using MQTT and smartthings are using home assistant as an intermediary, because home assistant has really good MQTT support. Here’s an example on that approach using the old architecture. So you might be able to set this up immediately, but it will stop working pretty soon because you’ll need to transition to the new architecture. But it will show you how home assistant might fit into the project.
Remember that in the past, people used groovy language DTH and smart apps and smartthings is going to pull the plug on those pretty soon. The replacement will be edge Drivers for the DTH and accessing the SmartThings API For the smart apps.
And just in case you need more information about edge:
NODE RED?
Other people have used node red, But I haven’t seen any project report on that in a long time and I know a couple of the Integrations, including the one using studio, seem to break over time. So to be honest I’m not sure where that option is right now, hopefully someone else will know.
Just remember that anything you find that is using Groovy will stop working pretty soon. you want to find things that use the new API or an Edge Driver instead.
(I know this post doesn’t really help, I just wanted to give you a few places to start researching. Hopefully someone will come along who has a current solution for the new architecture, and then I’ll just delete this. 
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