You could directly open the jp2 files in QGIS and then save them as GeoTIFFs. To prevent loss in quality, you just stick to the data type of the input files (can be seen in the raster metadata of QGIS)
on the different ranges. There is not necessarily a loss: Importing RTC S-1 into QGIS
You can compare minimum and maximum of input and output products in QGIS directly from the raster metadata to go sure the value ranges stay the same.
To my opinion,
gdal_translate -of GTiff input.jp2 output.tif
would be sufficient