Is there anything I didn't really notice that feels rewarding on higher levels?
At first, the game feels like it's deterministic with sprinkles of randomness, however, at high ascension levels it is the opposite - random chance until you get a broken deck that makes winning trivial. It's a slot machine.
I recommend watching runs from players that are going for win streaks. Many of the runs in win streaks are not slot machine decks. Going for a win streak forces you to take cards that you otherwise wouldn't have.
Watching runs in general will also show you that players win without broken decks. It's just that videos with the highest views tend to have broken decks because those are the most entertaining.
For me, the reward is having a deck that works well enough to tackle anything thrown at you.
This means many different things, but basically amounts to having the right amount of cards or relics that give you scalability for your attacks, a plan to mitigate attacks (with statuses or blocks), and a plan to handle beating the heart.
After a certain point, grabbing cards to beat the heart leads to beating other things easily - including Sword and Shield and the double Act 3 boss.
Funnily enough, I have the opposite problem. I'm often building decks that can beat everything easily except for the heart.
The thought that you put into strategy matters more when the game is harder. Personally, I don't find it fun past ascension 17 but if I play at a0 it's a bit mindless which is not as fun anymore.
Do you prefer being spoon fed rewards for grinding endlessly, like in your average free game?