I am working on a 5.3 integration currently. Will be published soon.
Does the Constraint Bones solver only work for ALS animations
Of course not! It is implemented for all UAnimSequences. 
Maybe your retargeting options are not correctly setup for your mesh. See get metahumans working · Issue #11 · tuatec/TTToolbox · GitHub or see this post IK Retargeter is not retargeting IK bone information - #50 by Sab_Rango.
We have to use Animation for the IK Bones. Otherwise the online retargeting does mess up the locations again. Also you should set your mesh as preview mesh other proportions might mess up the locations as well.