Home Assistant with Node Red running on a raspberry pi 3 b+. There is a lot of documentation on setting up home assistant. It took me no time to get Home Assistant running and a couple of days to setup my devices. It was really not that hard and now that it's done, the server just runs.
You just have to read the install instructions step by step. And use the Home Assistant Community if you have a problem. Then after Home Assistant is setup add Node Red and go through their tutorial. Node Red is very similar to stringify only more powerful IMO. Node Red has lots of nodes for things but for home automation you really only need a few of the nodes. I have all my Stringify flows now running in Node Red flows. Watch this users video, it helped get me started. https://www.facebook.com/haganwalker/videos/10213382077368166/
openHAB with Node Red is also an option. I just decided to use Home Assistant as it seemed to me a bit easier to setup. Getting smarttings to work with Home Assistant does require a signed certificate or Home Assistant cloud account. With Duckdns.org you could get the signed certificate for free and updated automatically. Or pay $5 a month for Home Assistant cloud. I went with the DuckDNS and it works perfectly.
I have Insteon, Google Calendar, LIFX, Rachio, Smartthings, Harmony Hub and Ring Doorbell & Chime all connected to Home Assistant. Those that prefer WINK, Home Assistant also works with that hub.
Since stingify is gone, and there is nothing we can do about that, this is the closest you will get. I've looked at the other options and nothing else I've looked at comes close. It does take some reading and time to get it all setup. If you are willing to put in the effort you will end up with a very nice alternative to stringiy.
Below is what my stringify flows looked like for turning on lights when a door opens and turning off the lights after a certain amount of time. I wanted the timer to reset if the door opened again and the lights were on. This required two flows to accomplish. The Node Red flow below does the same and was way easier to setup then using stringify. My flow required no additional programming code. Just setting up the Node and connecting the dots.
Node Red also has a great debug to test your flows unlike stringify.
Good luck on whatever you decide on.