Crystallisation problem
This molecule is an ammonium anion with an ester side chain as a chloride salt.

The first option you can choose to undertake is the vacuum evaporation of the solvent at room temperature or 30 degrees should prevent any heat degradation. I do understand your point about increasing the acid because in effect you have a buffer solution.

Your second alternative is to partition into a  warm amine-ophilic non-aqueous/organic layer and crystallize from there.
You can read more about that here: https://books.google.co.uk/books?id=YY3pCAAAQBAJ&amp;pg=PA8&amp;lpg=PA8&amp;dq=partitioning+of+quaternary+organic+salts+into+solvent+layer&amp;source=bl&amp;ots=O0iyoLglNF&amp;sig=jQb5hDP0SvhA2kg7CXGTXsIE4bg&amp;hl=en&amp;sa=X&amp;ved=0ahUKEwjAydrc5pjaAhXCIMAKHQI4ANgQ6AEITjAF#v=onepage&amp;q=partitioning%20of%20quaternary%20organic%20salts%20into%20solvent%20layer&amp;f=false>  

Also, see  https://onlinelibrary.wiley.com/doi/pdf/10.1002/jps.2600530804