Frequency band comparison between two groups

I am a beginner in signal processing. I have two resting eye closed data groups with group 1 having 40 data samples and group 2 having 100 samples. Both are taken from different devices with different frequency sampling rates (1024 vs 500), different number of channels(64 vs 32), different montage system (10-10 vs 10-20), etc but are eye closed.

I am comparing the frequency of average of all data samples across all channels in group 1 vs frequency of average of all data samples across all channels in group 2.

For that I have:

Step 1. For group 1 data(all 40 data samples): Did bandpass filtering for suggested frequency band, average re-referenced them around 32 channels.

For group 2 data(all 100 data samples): We dropped extra channels (converted from 64 to 32) keeping montage the same, cropped the data from beginning and end to make the time duration equal to group 1 data samples, resampled them from 1024 to 500Hz, did similar band pass filtering as in group 1, did not do re referencing as they were already average re-referenced around their 64 channels.

Questions: Instead of dropping 32 channels can we do some operation that converts 64 channels 10-10 montage to 32 channels 10-20 montage? Do we need to do re referencing again for group 2 data samples? If we will be doing wavelet denoising for each data in both groups, will we manually set wavelet, level and threshold for each sample or it will be same for all?

Step 2. Averaged all 40 preprocessed data for group 1 and converted it in a single data and similarly, averaged all 100 preprocessed data for group 2 and converted it in a single data.

Questions: Is averaging a good method, if not what other methods can we apply? What will be effects of averaging data samples before preprocessing vs after preprocessing?

Step 3: Comparing average of PSDs for both group's data (averaging all 32 channels) using welch.

Questions: Is average of power a good method to find differences in both group's frequency bands?