I am getting very low reflectance values (mostly < 0.15) in all bands of multiple Landsat 7 scenes after having converted to TOA reflectance manually in ENVI. These values are not supposed to happened. The objective is to perform relative radiometric normalization between these scenes (2010) and others from Landsat 8 (2016) (OLI scenes worked well!).
I suggest you use the Landsat surface reflectance product from USGS (some information here). While there are differences in how surface reflectance is calculated for each dataset, it is at least a fairly reasonable attempt at estimating the same magnitude.