Two options come to mind. Neither will damage the existing remote or require any wiring. 
Harmony hub
First, see if the harmony can learn the existing IR interactions. It can with a lot of devices. If so, you just have to have a harmony within line of sight of The receiver for the remote.
The harmony which integrates with SmartThings is the harmony home hub, typically sells for $89 at Best Buy. Sometimes it’s at that same price as Amazon.
If the harmony can learn it, that’s a good integration. I understand the issue, however, is that the current remote is a press and hold button, and it may be that the harmony won’t do that.
Push Microbot
A good alternative, if more expensive, is the push microbot from Naran. This is a tiny actuator intended as a “robot finger.” It can definitely do push and hold for pretty much any duration you want to set. I myself am quadriparetic, and I have one of these on a small personal blender to push and hold the start button, and another one on the microwave door button. Works fine.
If you just want to operate it from its own phone app, the push microbot by itself is $49.
But where it really becomes useful is if you add their $89 “Prota” Wi-Fi bridge, which also has its own IFTTT service/channel. Once you have that, you can use it with Amazon echo, which is how I use mine, you can use it with smart things, etc.
Setting it up the first time is a little annoying, but it works fine once you do have it set up. And it’s easy to write rules for it with IFTTT.
So it’s expensive, but it lets you retrofit a lot of devices which are not easy to retrofit otherwise.