Only the hard way of picking off individual discs
A word of caution , your Bach 333 individual discs will almost certainly be Unidentified so yo run the risk of missing Composition data and Credits as the single discs will not be in Tivo or MusicBrainz.
I would guess that in your Bach 333 folder the individual discs have some form of extra text like CD33 - A Bach work. If that is the case Roon will see them as individual albums and import them as such
DG spllit the box (indeed both boxes) into sub-volumes
So folder1 has CD1, CD2 etc Folder 2 has CD25 etc so a continuous disc count
I also guess your Mozart is in subfolders in much the same way again with “bald” CD1 etc no extra text.
One notable thing about these 2 sets and BTHVN as well is that the choice of performance is quite random, so eg the Piano Sonata set is not all by one pianist like Uchida in the older Mozart set. Hence a single disc almost certainly will not match any normal previous release hence it not ID’ed this is a bad thing.
Hence splitting off single discs may on the face of make life easier , not having the discs ID’ed is definitely not helping navigation.
The answer , well there isn’t one really but I would say the Mozart box is better than the Bach , if you can match up the individual sub volumes with MusicBrainz (You can BTW) you have best case.
The Bach , I would check the CD1 etc status of subfolders and group them as above ad it will probably import. I have both these sets but they ae so cumbersome I keep them out of Roon and use JRiver in parallel
This is the culmination of many unhappy hours at a keyboard but to me is what it should look like
Roon is working on a box set solution BUT where the split data comes from is a problem, where can you look up Disc subtitles like these above
If I can be any help do shout