Hi, not sure if you are still around Kayn as this is several weeks old now but just wanted to add my 2c. I would actually go down the Z-Wave route as opposed to Zigbee which was mentioned above. People have different views of course and I am speaking from personal experiences.
I have been on ST for years and in most of my experience Zigbee devices apart from the actual SmartThings own ones have caused me nothing but headaches and lots of Device Handler editing. It may not affect you being in an apartment but Zigbee devices have a limited range to the hub whereas Z-Wave devices that plug into a mains socket actually act as repeaters so create a sort of mesh network with the devices. I have also found the far more stable and they seem to reconnect back to the hub if there is an issue whereas with Zigbee I have had to jump through hoops and even reset devices on occasion. That being said Zigbee devices tend to be quite a bit cheaper than Z-Wave so this may play a part in your choice of items.
Be wary of the cheaper Zigbee devices from Aliexpress/eBay etc. and always search here for compatibility and Device Handlers for any device you may be looking to purchase. Again, from personal experience I have had very few issues with cheap Z-Wave devices from China compared to their Zigbee counterparts but that is my experience and others may feel differently.
Also, you mention in your last post about staying with ST and it being much more user friendly than Alexa. These two ‘systems’ are not really comparable, it is really intended that they both work together and compliment each other completely. You link your ST account to Alexa and most (possibly not all) your devices are now also voice controlled. You hopefully know all this by now but just wanted to mention it just in case you still had the same view.
With regards to security, ST is not really marketed or even designed/intended as a security system but with the right door and motion sensors etc, a decent loud siren or two, some tinkering and good routines you can have something quite adequate.
I used to use Arlo fully and still have a few dotted around but moved onto a grown up 24/7 4K system. Am sure you have already but if not then add the Arlo Cameras into ST. Not sure if you have heard of Bink, they are small battery powered cameras which connect over WiFi, the two AA batteries last for at least a ~year depending on the amount of times they are triggered. We used to be able to add these to ST but Amazon pulled the plug on that when they bought the Blink Company, however, I have heard rumours that this may be coming back soon. Point being, these a pretty good little cameras and if we get ST integration back they really cannot be beaten. I have them all over the place for extra security to compliment my wired system. I just got fed up with Arlo as it lacks major features like telling you if camera is offline, crazy lol. not to mention the battery life relly sucked, oh and the subscription fees. Maybe some of these issues have been sorted now but people were screaming out for years and Netgear did nothing about it (little rant over lol). Just take a quick look at the Blink XT2 cameras.
Soma do offer a Venetian Blind box as you say. I do not have one but have several Soma roller/vertical blind ones here. Soma opened up their API last year and I have them nicely integrated into ST with a custom Device Handler which I started to write and another talented member ripped apart and worked magic with Soma just released a version 2 device which is much faster than the original but this is probably of not much benefit with your type of blinds. Also you have no direct ‘manual’ control over the Soma units so it cost me another £24 to buy a ultra slim battery powered Z-Wave double toggle switches to control them with via ST. The costs add up unfortunately but am pretty happy with them. For Venetian blinds though I would look for units that actually fit in the top aluminium housing of the blinds. That way you can still use the rod as normal and they are also automated. Some of these are also Solar Powered/Rechargeable so you stick a panel to the top inside of your window and no need for power cords etc. Not much on here about blinds for some reason (well was not when I searched a while back) but do have a good search on here and also google for any advice about V Blinds and ST etc. Will add though that as there is not a huge market for these types of devices they are not cheap as you have seen with the Soma ones. Let me know if you do find a decent V Blind unit that works with ST, I could do with a few.
As mentioned, have been here a pretty long time with the occasional break and all on here (bar a few lol) are great people and more than happy to help so just keep reading and more importantly asking questions if you need help with anything.
There ya go, had a little time just now so spent it here. Hope a little of it may help you.
Hope you all stay safe and well out there in these crazy times!!!