I find the most fun in the details at the level you’ve now reached.
None of these are free but are mentally stimulating enough for me to enjoy the tuning and quite modestly priced I think.
Keep the BHK for nothing other than the ability to try different sets of early 50s-early 60s NOS <$50 per pair 12AU7AW/6189/5814WA tubes. 1955 GE triple mica black plates are my current source of je ne sais quoi.
Convolution filters in Roon can be quite interesting. The effect/experience with different genres/albums/tracks. Highly recommend HAF https://www.homeaudiofidelity.com
Oh, and the DS. Beyond different fuses and power cords, 3M makes the AB5000, AB6000 and AB7000 series RF absorbing lines of non-conducting stick on material. Stack, leave the backing on, a few 1"x4" sheets on either side and between the RAM modules in your server and to the locations in the DS that Ted refers to in this post from a couple years ago.
The material is expensive but odd packs of qty 100 or 250 1.3" x 4.25" will appear on Amazon for like $17 instead of the more standard $287 for the same sq. in. It’s fun to experiment with and safe.
In general with DACs in a system you should be more worried about RF coming out than going in. The case certainly shields quite well but what about the RF soup inside the box? Selectively absorb it to change it’s effect on other components that oscillate at a different frequency.
Isolation and damping help/change the BHK quite a bit. I think of it as tuning the harmonics produced by the tube filaments.
I prefer a combination of spring isolation, a decent mass bamboo or plywood plinth and then damping footers under the component. None of these need be expensive. Different types of wood blocks as footers change things too.
I don’t find upsampling helps or hinders the DS in anyway. I don’t bother with it at all since Snowmass let alone Windom. HQPe made slightly different flavors of great sound but nothing as significant as some of the other tweaks above.