I was in the same situation as TS some time ago, when I started. But then I found, that there are many nice products on the market, which you just need to find.
So if you need cross-platform customizable UI without any programming I know at least two options. They both provide responsive feedback due to MQTT nature(no REST stuff). All you need is to set up MQTT event bus binding in OpenHAB and …
- CommandFusion iViewer - for this you will need MQTT Plug-in, which I provided in it’s group. After that use GUI Designer to draw any layouts and create commands and feedbacks to communicate with OpenHAB. Works on iOS and Android, free for one page, for multipage you must pay something.
- The Home Remote - this is even simpler, as MQTT support already included. Again, draw everything in Designer Program on PC. Simulate! with a click. It has apps for iOS, Android and Windows 10. Free.
In both cases programming is limited to set up correct MQTT Topics and value processing. Benefits of both solutions - they will work not with just OpenHAB but with any automation system supporting MQTT, like Home Assistant, or ioBroker, so if you migrate there is no need to change the GUI App.
I use CF at home, and Home Remote on iPhones (connected via CloudMQTT Broker to my OH). No ports forwarding. All SSL secure. Instantaneous reaction (in 3G Network offcourse)