There are some options for Zigbee, but they are far less mature than Z-wave at this time, and it does require more hardware. I personally only had a handful of Zigbee devices, and frankly two of them were ST branded motion sensors that gave me false positives periodically. I decided to move to Z-wave and gave away/sold all my Zigbee devices. I don’t know how many devices you have, so that’s something you’ll need to decide.
True, there will be some growing pains as you learn to set up openHAB the way you like it. Things like modes that don’t exist by default in openHAB, but the nice thing about it is that you can define them. I actually set my openHAB up in a similar fashion to my old SmartThings setup with a mode String item that can be set to home, away, or night.
I initially planned a gradual switch over to openHAB, doing one room at a time and creating rules that emulated my SmartThings routines. Once I got my Z-wave stick, however, I got so excited with the possibilities that I decided to completely switch everything over and gradually work up to a mature installation. One way I did that was with multiple sitemaps. I created a "guest’ sitemap that only did the basic necessary functions like the main light switches, without worrying about color temperature and other settings. That way other family members had access quickly while I built out the rest of the system.
Once you decide to move a device over from ST to OH, you’ll need to exclude it from ST before including it with your Z-stick. The Z-stick is nice because you can carry it around and bring it to hardwired devices if necessary. Pair them in the order you want them to show up in your interface; the stick assigns node numbers in order as you associate them.
Once you have them paired, plug it into your openHAB computer and I would restart.
From there, I’d read up on setting up the Z-wave stick. You’ll need to determine the port your serial stick is using and set it in PaperUI.
https://docs.openhab.org/addons/bindings/zwave/readme.html
Good luck!