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

I have not heard the Grimm and have no POV.  I would bet the Oladra is a better compare.  It is definitely better than the K50 on a technical basis.  

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.  

I like the build quality of Aries Cerat better. I have liked the sound of systems with Aries Cerat DACs in them better than when I have heard the Lampizator. That said, I personally have never done a compare of the two in the same system.

Regarding server, when you see a device is "Roon Ready" like the Mola Mola it is a DAC and Renderer/Player when you connect to it via ethernet. This means that the renderer/player duties are not being handled by the Nucleus. This is optimal as this stage does not require a lot of processing power and noise can be introduced with too much processing power.

When you connect via USB, the Nucleus is performing as player and server. This is where it tends to underperform and sound mediocre. If you have a USB cable lying around, try connecting the Mola Mola to it via USB.  I bet you will hear a significant degradation in sound quality vs your current ethernet setup. 

The Nucleus is a stunning value when used as a server alone.  Best upgrade from a Nucleus for people connected via USB is not to get a higher end single box server but to get a separate player renderer.  

This is because even the best servers are not at their best when performing both tasks. It is just the nature of Roon to perform best when server/roon core are on a very high powered computer and the player is on a medium to low powered computer. This is why Antipodes adopted a dual computer approach and why Pink Faun recommends two units for absolute best performance.

That is correct.  The Oladra or K50 have two computers in one box (with separate power supplies) along with an elite clock.  

My understanding is the Pink Faun Ultra is very, very good but their top performer is a "double ultra" which is $47K which is one Ultra as Server, one Ultra as Renderer.  With Pink Faun, it is all about the clocking and they have superior clocking to just about everyone.  I am considering adding Pink Faun as a brand but haven't personally demo'd it yet.  

I work with the North America importer for Aries Cerat and Pink Faun and I know they work well together. 

If the Lampi Pacific has a bridge available that is great.  I do not know how it compares to the others mentioned above.  I will say this.  I feel that Antipodes K50 player and clock outperforms the Weiss Player in the DAC 501/502.  It may be that that K50 (or Pink Faun, etc..) might outperform the network bridge in the Pacific.  

You are absolutely correct.  Move to a DAC that is not Roon Ready and using Nucleus as renderer and player might actually deliver a regression in sound with the upgraded DAC.  These are the best players I have heard in descending order in terms of price:

Antipodes Oladra or K50 - these are server, player and clock solutions that are not inexpensive but will deliver elite levels of performance.  This is the sort of elite solution that would be at home in your system.  $25K or $17.5K respectively

Antipodes K22 - Clock and Player portion of K50 and can be paired with any server.  $9K

Weiss DSP 501/502 - this unit has a variety of DSPs including room correction.  There is an argument simply adding this in the chain and applying a DSP would resolve your issue with 100% certainty as either the d'esser or vinyl mode would likely add the warmth you are looking for.  $7.5K/$8.5K

Bricasti M5 - this is a pure player and is an awesome value.  Nothing fancy.  No DSP.  Just a high quality player.  $2400

Chord 2Go/2Yu - This is particularly interesting due to its high level of flexibility and wireless capabilities.  That being said, it is not at its best via Wireless.  It is at its best wired and is equally good compared to the Bricasti M5 via Coax though harder to setup.  $2240

I have not had the two units side-by-side to listen.  I am hoping to do this later this year and also get a demo with the Taiko at a customers house.  
 

you are spot on.  Two computers in Oladra vs better clock in Pink Faun.  You also have a great software solution with Oladra matching Roon Core with Squeeze Light.   Alternatively, Pink Faun had a proprietary software that is an upgrade vs Roon.  
 

These are VERY similar.