*Scribbles notes*
Bloviating further because I'm an old blowhard in love with the sound of his own keyboard...
Regarding the smaller lab ship and keeping variants in mind, a design which is "mission configurable" would fit a larger number of uses. Yes, while the original lab ship has big honking lab spaces the referee could say was full of this, that, and whatever, it would be nice to have had a few "mix & match" deck plans of various lab spaces or ship missions.
For example, while the labs in Death Ship would have been "boring" chemical and animal handling/experimentation types, what about a ship operating ROVs investigating a gas giant and it's moons? Might the floor access plate in the main lab be replaced with a drone deployment port and part of that lab dedicated to drone handling, repair, and storage?
Riffing on CT's "Suleiman's in service carry varied equipment" and TNE's "cans & sockets" examples, a small lab ship with a 30dTon cutter module bay would be a way to present a deck plan with a myriad of uses. A lab ship meant to investigate this, that, or whatever would carry a this, that, or whatever cutter module as necessary.
Using cutter modules means the deck plan could use previously published modules too. GT even has an entire book devoted to them. The ability to carry module means that surplus or second hand vessels of this type might not be working as lab ships at all. Such ships might not be perfect or even efficient traders, freighters, or liners but sometimes any ship is better than no ship at all.

IMPORTANT NOTE: The above is not my idea. I am not creative. I steal and modify instead. My "thinking" about this was entirely informed by Walt Smith's superb Jump Pod design.
Finally, regarding the Leviathan, I neglected to mention that MANS, the Marches Naval Auxiliary Service, operates variants of that design too. Do the IN, subsector, or duchy fleets use them? What roles do they fill? What changes were needed?