I use registax 5 for Uranus (In my opinion, it is more reliable for single-point stacking of small planets).
After wavelets i make correction of diffraction artifacts on the limb (in the Photoshop mainly). It is necessary. You simply cannot make raw artifact-free image of Uranus with 300 mm telescope. First diffraction ring is very bright (especially on the instrument with large central obstruction, i think).
Before stacking i make dark/flat calibration, 2x resampling and cropping of SER file in one self-made utility.
Stanislas, it is unclear by now, how spots are moving and whether they even visible in R/NIR. I think, we need some undoubtful images from large instrument (or may be more good images from small). To be honestly, i don't see how it may be possible to catch spots visually if they are not visible on the R/NIR picture which is taken in excellent conditions.
Franly I cannot answer about your question precisely.
What I can answer is on last 14th with the 254mm the spots were obvious.
On the 16th I started with the refractor, the planet being around at 26° elevation and then change the bigger scope if this will be still observable but with a better contour and brightness. The exercise was not possible because after the moisture deposition the sky conditions became worst by the seeing.
You know with such apertures, moderate, there is a world between "seen" and resolved. The answer could be here, this mean the contrast as you know faint but accessible still.
Now the move of the spots can be predicted as we know the atmospheric circulation speed versus the planet lattitude. Few (°) of drift may be necountered per day.
This is a challenge for determining the speed rotation with such very dim features, on short periods (1 or 2 weeks) hard to do with an acuracy.
When we catch these brightness discontinuities visually we are in a certain way satisfied of the results.
Persevere in your works on, this shall be over a greater period you will conclude something.