If you don't like the humanoid thumb solver because there is not enough degree of freedom, simply switch the thumb to generic animation.

The humanoid has a set a required bones like hips, head, foot, hands, etc and some optionnal bones like finger.
Optionnal bones can be either animated with the humanoid rig or as generic FK animation.
You can choose to animate the thumb with FK or humanoid while having all the other fingers animated as humanoid.

To do so you need to follow a few step:

First unmap the thumb in your avatar configration file.

By default the importer only import humanoid curve when the rig is set to humanoid, to import additionnal transforms you need to create a mask and check all the additionnal transforms that you want to import.