Spline retargeting might be a way (Character Object> Mixamo Rig)
Reducing Joint Amounts:
This is not a trivial problem, and I can only hope that many people write a suggestion to Maxon to allow for a Mixamo export option.
Open a support ticket> Suggestion
One problem can be explained with the overall length of an arc, as a spine can form an arc. Thinking of an arc with an equal radius, it matters how many segments are used. The summary length of all segments will vary based on the number. Think of a circle represented by a triangle or an n-gon with 64 segments. (The near circle-like spline will be around 20%+ longer).
This excludes just to average the center joints and apply the value to the center joints for the low joint solution.
This and several other problems are perhaps of no importance in one or the other case: An Artist call.
Another option, more a brute force, to reduce every second joint to zero length while compensating for the loss of size with the previous. Not ideal either, but a quick fix. For moderate changes, that might work (?).
Joints typically work with rotational values only. The position where that rotation happens is crucial for anything that follows after that.
I have attached a scene to showcase the problem, going from more (5) to fewer (3) segments. This is in no way thought of as a solution, more to initiate brainstorming for specific cases. The file introduces a stretch and squeeze, which might not be wanted at all.
Click thumbnail to see full-size image