I don't have the time right now to understand your code, but I think Sow[] and Reap[] might take care of your memory issue.

Here is a overly-simplified example of how Sow and Reap can be used in a situation like this:

f[x_] := Do[
   With[
    {
     r = RandomInteger[i]
     }
    ,
    If[OddQ[r], Sow[r]]
    ]
   ,
   {i, Abs[x]}
   ];

g[x_] := Reap[
   Do[
     With[
      {
       r = RandomInteger[i]
       }
      ,
      If[EvenQ[r], f[r]];
      ]
     , 
     {i, Abs[x]}
     ];
   ] [[2]] // First;;


g[100]

This example wraps the outer Do[] with Reap[] which will collect only those values sowed by Sow[] in the inner Do[].

As I understand it, this technique will only retain in memory the values that you want to keep while immediately discarding those that fail the test.

Good luck, and have a great and safe rest of your weekend.