I have a similar project: For my fathers boat, I want to use a Nextion as a central informations display. I will use a asr6501 as self containing gateway/controller, so maybe we can exchange experiences.
I have been using OpenHAB and MQTT for a number of years now.
The reason for using MQTT and not just a serial connection from the MySensors GW is partly the flexibility as many have stated previously here. Easy to test, using e.g. the mqtt.fx tool. If you want to know what is going on you simply connect to the MQTT stream and "snoop" on the messages.
Another reason is that I am running three geographical sites on one OH installation. OH is running on a NUC at my home. There is also an MQTT GW here. Two other My Sensors MQTT-GW are running remotely in two summer houses. The three GW connect over internet to a cloud-based MQTT broker and OH connects to the same broker. In that way I do not have to open any ports into my home network for access should I have had a local MQTT-broker at home.
There are also some very cheap usb-stick size SSD's now. Now sure about the long term quality, but it's.. interesting.
This 64Gb one is 15 dollars including shipping
This 128Gb one is $20.
Finally, this Kingdian one seems very popular. It's $30, but is a bit faster.
There doesn't seem to be a ton of interest in this but I wanted to provide an update. After writing the transport and using it for a bit I've decided to go another direction with the integration to SmartThings. I felt like the custom transport would require more maintenance and I wanted more of any option that was configuration rather than coding. Because there was C++ code on the MySensors side of things and then Groovy on the SmartThings side it just seemed like a brittle integration with the design and how I wrote the code. SmartThings is going to be moving in another development platform direction as well at some point.
I'm working on an MQTT solution with Mosquitto and Node Red (on a Raspberry Pi) and it will also require some Groovy on SmartThings but a much simpler solution IMO. There will be more to come and I'll share here when it's complete but I do have a prototype/proof of concept working. The two way communication is much better/cleaner IMO than how I did it with the transport. If this is something that interests you let me know and I'd be happy to collaborate.