Based on the little information and measurements I found about the RE-Zero it seems to be an IEM with relatively forward upper mids and rather flat bass response.
That should be a rather neutral sound signature, but with some uneven treble spikes.
I'd look for a smoother response in the region above 2kHz, and a bit more sub bass, which seems to be recessed on the RE-Zero
If you want to stick with dynamic driver IEMs, the two main contenders IMO are the Tanjim Oxygen and the JVC/Drop FDX1.
Both of them have more bass boost and a smoother upper midrange, and should be a significant upgrade technically.
I have personally only heard the Oxygen very briefly, so can't say much, but I have the FDX1 and it's a versatile, fun and technically pretty good option, definitely great value.
If you want a bigger step up in SQ I'd look at the Thieaudio Oracle, more info here:
https://crinacle.com/2021/07/27/the-best-500-tribrids-oracle-excalibur-variations-est112-ej07m/
I have also not heard it, but I have the Variations against which the Oracle is being compared.
The Variations is excellent, but the recessed lower mid/upper bass region makes them a bit thin and lean. The Oracle should be close in technicalities and have a much safer tuning. Compared to your RE-Zero I would expect a significant increase in detail, clarity, separation and staging.
You might loose a little bit of body to the music, making it sound a little thinner, and vocals might be less intimate than what you are used to.
As always it would be best to test before buying
Also consider that you might need to invest some time to find tips that provide good fit.
Tips and fit can have a pretty big influence on the sound signature.