Hiding the 無し form.

@@ -8,0 +9 @@
+<ke_inf>&sK;</ke_inf>

No problem.

@@ -16 +16 @@
-<gloss>when you would be a filial child, your parents are already gone</gloss>
+<gloss>when you are ready to be a filial child, your parents are already gone</gloss>

If you're willing to drop the "want/wish" of したい, what do you think of:
  when you're ready to be a filial child, your parents are already gone

GG5: 孝行のしたい時分に親はなし． When one would be filial, one's parents are gone． ｜ A son is always too late to think of his parents.

I feel the second gloss is overdoing it a bit. Trimming.

@@ -16,2 +16 @@
-<gloss>when you want to become a filial child, your parents are already gone</gloss>
-<gloss>when you finally appreciate what your parents have done for you, it's too late to repay them</gloss>
+<gloss>when you would be a filial child, your parents are already gone</gloss>

孝行のしたい時分に親はなし
https://imidas.jp/proverb/detail/X-02-C-10-1-0018.html#:~:text=こうこうのしたいじ,はいないことをいう。
親の苦労がやっと分かって、これからは親孝行をしなければと考えるころには、すでに親は死んでこの世にはいないことをいう。

Just made an edit to 風樹の嘆, and pointed it at this entry.

"when you will wish" was not grammatical, which prompted me to edit to, something....  

Gloss #1 aims to be literal. I don't think that "want to be filial" in isolation makes a lot of sense in English.  I have a similar reluctance toward "I want to show filial piety", I feel like an indirect object is needed.  Another possible literal gloss:

  when you want to show filial piety to your parents, they're already gone; 


For the second gloss, I've taken the imidas explanation(when you understand the hardships of your parents...) and tried to arrive at something more relatable/analogous to Western experience (since we don't really do filial piety here...).  "It's too late to repay" tries to capture "you want to repay", which is the 孝行のしたい bit. It's ends up being ambiguous about the parents being dead-dead, but technically I think the original is as well?  "It's too late to thank them" seems less ambiguous about them being dead, but "thank" could be a one-off gesture, and not a change in behavior (which repayment would require). 

Well, best I could come up with.  I won't take it personally if the gloss isn't kept.

@@ -16 +16,2 @@
-<gloss>when you will wish to be filial, your parents will already be gone</gloss>
+<gloss>when you want to become a filial child, your parents are already gone</gloss>
+<gloss>when you finally appreciate what your parents have done for you, it's too late to repay them</gloss>