You didn’t say what country you are in, and the device selection does vary. 
If you are in North America, Zooz (the house brand for the smartest House) has a popular in-line relay with good smartthings integration:
They also have staff who participate in this forum if you have any questions
That said, it’s very important to understand that any mesh network, such as zigbee or Z wave, cannot enforce sequencing. Messages can and do bounce around the mesh and may take different paths. So two messages sent very close together may arrive in the opposite order from what you expect. Most zwave manufacturers say they cannot guarantee sequencing for anything closer than one minute apart.
That’s not to say that community members don’t use these devices for those kinds of use cases. For example, there are quite a few lighting systems which require pattern presses, like three quick presses followed by one long press to change color or something similar. Often the pattern has to be completed in less than five seconds. There are people who have been able to set up this kind of pattern using Webcore and find that it is mostly successful, although every once in a while it may fail.
I just mention that because if there is a safety issue if your particular pattern fails, then I would not try to use a mesh-based device. Instead, look for Wi-Fi, which does enforce sequencing because it’s continuous connection rather than mesh.
BTW, if you’re not familiar with Webcore, it’s a community created rules engine which is essentially a scripting language for smartthings. Very popular for community members with good technical backgrounds. They have their own forum and lots of people who will be glad to help you set up “pistons“ (that’s what they call their rules) if you get stuck. while the official smartthings features generally follow the one minute rule, Webcore has many more options.