I recently switched over to TickTick after hearing that Microsoft are discontinuing with Wunderlist which I've been using for years. For the last 3 months I've been using TickTick, I've been impressed. Before that I tried all the other major to-do apps but they didn't suit my needs (esp. folders, subtasks, design, cross-platform support etc).
TickTick is eerily very similar in UI design to Things and supports dark mode. They also support MacOS which was essential for my needs and as I'll be moving over to Android soon after being with iOS for many years, Android support was essential.
The only downside to TickTick so far for me is that they have a subscription model unlike Things. Usually I despise the use of subscriptions but it's an app I genuinely get good utility out of that it makes it worth it. Would definitely recommend others to give it a try.
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I'd recommend giving TickTick a try. Similar structure to Wunderlist and supports all three platforms. It's free, but has a paid option for certain features.
Alternatively, try Microsoft To-Do which will eventually replace Wunderlist. Only problem with that though is they don't officially suport MacOS.
If I was not exclusively Apple, I'd go for TickTick too, it's a great app.