Ok, I've got something. Looks like it may have a conflict with the new version of GiddyUp:

Exception loading def from file Creation_Brute.xml: System.ArgumentException: Could not find type named GiddyUpCore.CompProperties_Mount from node <li Class="GiddyUpCore.CompProperties_Mount"><isException>true</isException></li>
Parameter name: node
[Ref 58BD1051]
at Verse.DirectXmlToObjectNew.ResolveTypeForNode (System.Type defaultType, System.Xml.XmlNode node) [0x0005a] in <cb53cd4422904947932d33f561ad8d15>:0
(wrapper dynamic-method) Verse.DirectXmlToObjectNew+DummyTypeToHoldDynamicMethods.ParseAndSetListField_Verse_CompProperties(object,System.Reflection.FieldInfo,System.Xml.XmlNode,System.Type)
(wrapper dynamic-method) Verse.DirectXmlToObjectNew+DummyTypeToHoldDynamicMethods.ParseAndReturnDef_Verse_ThingDef(int,int,System.Xml.XmlNode,System.Type)
at Verse.DirectXmlToObjectNew.DefFromNodeNew (System.Xml.XmlNode node, Verse.LoadableXmlAsset loadingAsset) [0x00148] in <cb53cd4422904947932d33f561ad8d15>:0
- PREFIX Dubwise.DubsBadHygiene: Boolean DubsBadHygiene.DubsBadHygieneMod:Prefix(XmlNode node, LoadableXmlAsset loadingAsset, Def& __result)
- PREFIX Dubwise.DubsCentralHeating: Boolean DubsCentralHeating.DubsCentralHeatingMod:Prefix(XmlNode node, LoadableXmlAsset loadingAsset, Def& __result)

Possible Matches:
[Source: RimWorld - The Dark Descent (Continued)]
[File: C:\Games\Steam\steamapps\workshop\content\294100\3312022150\1.6\Patches\GiddyUp.xml]

Same error for the engineer, grunt, goliath, harvester, hound, tesla, and wretch. ("Possible Matches" provided by the Stack Trace mod)

A little further down in the log, there's this message: "Caught exception while loading play data but there are active mods other than Core. Resetting mods config and trying again.", followed by a nullreferenceexception thrown by Alpha Genes.

Shortly after, this message appears: "Could not recover from errors loading play data. Giving up.", followed by another nullreferenceexception, this time triggered by "Bigger Gravships (and other Odyssey spaceship related settings)"

Dropbox link to folder with player.log and player-prev.log:
https://www.dropbox.com/scl/fo/2yqpieuhaen0nl49dl0wz/ACbdKwxRjwoGKQx_yz6-viw?rlkey=wakper4bvg6c0gsak4kfdzmac&st=dd1p9045&dl=0

Player.log is working, without Dark Descent mod installed. Player-prev.log is not working, with Dark Descent mod installed. Whether the mod is installed is the only difference between the runs, though I did end up killing the process with DD installed because it got stuck for 30+ mins at the last few lines. I used BeyondCompare to find the issues above.