What will be the possible conditions to react bromo alkane with diethyl phenylmalonate?
You have tried a number of sensible approaches and nothing is working well. I think the problem here is that the anion you are creating is very stable and hindered and no amount of tweaking will significantly improve it. You either accept the yield as it is and scale the reaction accordingly, or change your strategy.

Your next step is hydrolysis and decarboxylation to give a substituted phenylacetic acid so I suggest you start with ethyl phenylacetate. You could alkylate it with NaH (1eq) in DMF with KI then 1-bromopentane. That should alkylate well, but there will be a percentage of di-alkylation so I'm going to suggest you condense ethyl phenylacetate with pentanal and then hydrogenate (or otherwise reduce the double bond). I think this will give you a cleaner reaction mixture and higher yield.