Which DACs are known to be sweet/rich/relaxed?

I’m going to zero in on something else in your system. The cabling —which is actually the cheapest component in your system, not the DAC. Your brand is a great bargain for SQ, even for systems costing much much less than yours. Their power cords and speaker cables are nice for the money and I use both. But the interconnects do not seem to perform the same…but are certainly ok for their price. Some cables can perform differently system to system, but in my system, these ICs produced unclean/noisy high frequencies and I can see how this might contribute to your issue… A high end, high resolution system deserves very clean and clear signal cables. Not to mention some absorption somewhere! Even if just at first reflection, you may see a drop in glare that you don’t even know is there.

A curse of a highly revealing system.
How long is your USB cable from Roon nucleus to DAC and what cable cable are you using?

Have you tried stock power cords on amps and DAC now that you lived with the TWL loom for a while?

My experience is that if you use a Nucleus or Nucleus+ as server & player, it tends to be mediocre.  Just as a server (which I assume is what you are doing with Mola Mola), it performs at a very high level.  The issue is the processor is too powerful to serve as a good player.  

Where I find there is significant improvement is if you introduce a master clock like what you see from Antipodes or clocking between server and DAC can be synchronized.  This happens with I2S connections or when there is a dedicated clock connection.  

My most common upgrade in servers is customers who go from a Roon Nucleus or Nucleus + to an Antipodes K50.  

You are on the right track with Aries Cerat or Lampizator.  Both are very, very good though I tend to like Aries Cerat better.  Personal taste though.  

The obvious other two options to me are the MSB Premier and the Playback Designs MPD-8 Dream DAC.  There is a slight sweetness to both with a complete lack of brightness/sibilance that is quite shocking and enjoyable.  

Theoretically, those four units should do the job and will all be a touch warmer than the Mola Mola while delivering the same of better levels of detail.  

@jriggy you are absolutely right, thanks for zooming in on that. I tried Nordost Valhalla 2 speakers cables and interconnects. Different but not per de better. Later I found out these cables are on the bright side.

also tried Tellerium Q, one level below top of the line but not interconnects. Sounded good but never pulled the trigger.

Any suggestions for cables that match well with the rest of my system? While keeping in mind my goal to make things a little smoother.

@verdantaudio thanks for your recommendations! Worth looking into.

I however don’t understand you remark about using a Nucleus as a server & player vs. just as a server. I don’t have local files, I play/stream from Tidal using Roon. My Nucleus is connected to a wireless access point with an ethernet cable. And my Mola Mola Tambaqui DAC is also connected to that same access point with an ethernet cable. I don’t use USB.

What makes you prefer the Aries Cerat (Kassandra?) over the Lampizator (Pacific)?