actually you need the resampling only once at the beginning of your graph and the output can be used in the successive steps.
And if you only deal with in your chain with the 10m bands it adds no, or at least only a little extra processing time, because, as you already said, it is a no-op.
I tried the use case you have described in different ways and in 5.0.x and 6 BETA versions.
In the Graph Builder, on the command line and doing the steps separately in the GUI. And it works.
It is not slower then simply removing all bands and masks from the product. This can be easiest done, by unchecking from the context menu of the product the option "Group Nodes by Type". Afterwards you can select all the nodes you don't want and hit DEL-Key. Then you can export the product with out resampling. It takes the same time as when doing Resampling and BandSelect in the Graph Builder.
What slows done the export is the format. Big GeoTiff is slower.
For my scene it took:
DIMAP: 76 seconds
Big GeoTiff: 4 minutes
GeoTiff: 80 seconds
If you only want to have these 4 bands you have mentioned, you can use the 'normal' GeoTiff format, because it doesn't exceed the 2GB barrier.
I've attached my graph which I have used on the command line with gpt.
S2_Resample_BandSelect.xml (1.5 KB)