I have recently been looking at a curtain system for the UK but with slightly different criteria but since this thread has sprung back in to life I will add my current findings.
Aqara do one that uses Zigbee and they also do their own hub. This is UK compatible and available but they are in the process of releasing a new M2 Hub so I would hold off and wait for that. They can be connected to Smartthings and they can also be connected to HomeKit however I am getting the impression that to use HomeKit puts it in a mode that precludes other connectivity like Smartthings at the same time. This is unusual and would seem unnecessary as the Philips Hue hub for example has no problems doing HomeKit and other connectivity at the same time.
I personally find the whole Aqara product line a confusing mess for this and other reasons and as long as the next option evolves that way I expect I will not be using Aqara myself.
Slide is a newish startup. They are European and make a smart curtain system and I believe already support Alexa and are in the process of adding Google Home. I am unsure about Smartthings.
They are intending to add HomeKit support. This is the system I am currently considering myself.
Somfy are a French company but also sell in the UK. They do have a smart curtain solution which uses their own proprietary 433MHz RTS system but there are some integrations listed in these forums to link it to Smartthings. They did at one point promise to add HomeKit support but now appear to have pulled out of this. I am therefore excluding them from my own considerations.
There are several systems listed on Amazon but at least as far as HomeKit support cheat if not out and out lie. The way they do HomeKit and possibly other integrations is to have a separate smart plug e.g. EveHome and when that plug cycles the power this is detected by their product and causes the motor to activate. If closed it opens, if open it closes. This of course means you cannot open to say 50%.
There is an ultra high end and hence hugely expensive Lutron solution.
Be aware, a lot of discussions even if the word curtain is included will misleadingly actually talk only about blind only solutions. Soma for example only do blind solutions and no curtain solutions. Similarly so far Ikea only do blinds.
Also be aware that the US calls curtains ‘drapes’.