This is one way you could go:
-Buy one of the WiFi-IR Remote IR Control Hub flash it with a modificate firmware I don't know if one of the device on the HubDuino from @ogiewon could get you there GitHub - DanielOgorchock/ST_Anything: ST_Anything is an Arduino library, sketch, and Device Type that works with your SmartThings ThingShield to create an all-in-one SmartThings device..
You could also use tasmota @markus [RELEASE] Tasmota for HE - Auto-detecting Tasmota drivers + Tasmota firmware 7.x/8.x for HE (for use with Tuya, Sonoff and other ESP devices)
this 2 firmware speak directly to the hub and are very reliable I use them both at home.
-then teach the hub one of the ir commands that the harmony produce to control the devices.
If this won't work I know you could flash regular tasmota on the IR Control Hub and connected to an MQTT broker but you require a broker if you don't have one, and I promess make a driver that talk to that broker and generate events on the hub for you then [Rule Machine] from there.
This is the beauty and the reason why we ran from ST to Hubitat here we have many options
and @tlinsenmeyer welcome to the family
