Does enabling LPIM work even if the memory pressure is coming from SQL Server itself
Does enabling LPIM help in this scenario since i see IndicatorProcess=2 in most of the cases.


No.


  I already have 128GB RAM.Is that the maximum i can have for SQL Server 2016 Standard.?


Yes.

And


  I see that plan cache gets cleared often when i have more usage
  and
  most of [the frequent queries] runs fast when there are plans available in plan cache


Suggests you should focus on your plan cache.  Are there a large number of single-use plans?  If so consider setting the optimize for ad hoc workloads configuration option.