If you already have added protease inhibitors (there are plenty of cocktails available) then it may not be degradation but failed read-though during translation and early translational termination. You see this quite frequently in E.coli with eukaryotic genes, even with codon+ strains.

Try richer media and lower temperatures for induction (or even auto-induction) and try Rosetta/2 (DE3), or similar,  in place of codon+ as they can help with a wider range of rare codons.

At least you have part of your protein-many times with GST you see only the GST produced  as the translation stalls as soon as it hits your codon-biased eukaryotic sequence.

You may need to add at least one more type of chromatography to the purification to remove impurities, if the problem is contaminating proteins rather than read-through or proteolysis.

I have grown it  at 18c too for 4-6 hrs and string washing also.