Current situation:
|1.g4 2.j15 3.q14 4.p11 5.l12

Imho, either we threat to cut in the center and then try to open a path to the lower right, either we try to block white on the top.

I see only two possible moves :
6.n15
6.l10

If !6.n15 7.o13, then 8.q18
If !6.n15 7.u14 then 8.q18 again, but after 9.u19 I don't know if black can connect (I doubt it)
If !6.n15 7.u14 8.o13 but after 9.s9 I don't know if black can connect (I doubt it too)
...maybe if we attack from both sides ?

If !6.l10 7.h10 8.l14
...and I cannot see any better answer (but I'm just as you, Maraca... having some difficulties to be critical enough at my moves...).

So I think that I'd prefer 6.l10...

Yes, those two moves are the most reasonable in my opinion too. There is also a possibility to play a move in the O9 area.
|1.g4 2.j15 3.q14 4.p11 5.l12

6.l10 forms a nice pattern with J15, but we are a little bit weak on the other side, the Achilles this way is not that good.
An example of exploiting this weak pattern would be something like 7.j11 8.j9 9.h10 10.h8 11.f9 12.f7 13.d8 14.d6 15.o9 16.n11 17.p12

6.n15 is my preferred move. It has to be answered and it makes P11 stronger to connect to the right. I would leave the top open and play a move there later.
Interesting answers are 7.u14 and 7.s13, because 8.q18 won't work then (9.s18). We could go for a move like 8.p7 or try something on the bottom like 8.q19.

|1.g4 2.j15 3.q14 4.p11 5.l12

I was wondering if 6.m14 would work:
7.m15 8.n16 9.k14 10.l17 or
7.l13 8.k13 9.n14 10.m12 11.p12 12.n10 13.q10 14.q7

otherwise I guess the Achilles spoted by Maraca force to abandon 6.l10 toward 6.n15

2nd line is faulty, it may be repaired by 7.l13 8.k17 9.o15 10.n16 11.n13 12.p17 13.t17 14.r17
But yes 6.n15 has more influence on the right.

Yes one refutation of 6.m14 is 7.p12 with possible continuation:

8.o9 9.q10 10.q8 11.n11 12.l8 13.m9 14.n7 15.k8 16.j7 17.i7 18.h6 19.g6 20.f5 21.e5

btw, l10 isn't a clear win for white at all, this is how it could go:
|1.g4 2.j15 3.q14 4.p11 5.l12 6.l10 7.j11 8.j9 9.h10 10.h8 11.f11 12.g6 13.o9 14.n11 15.p12 16.o13 17.q10 18.p18 19.t17 20.s21 21.q20 22.r19 23.n17 24.m17 25.l16 26.o18 27.o15 28.k16 29.f16 30.g20 31.i18 32.h16 33.h17 34.i21 35.k19 36.l21 37.o19 38.n19 39.m20

Actually I tend to favour black, L10 could have been an interesting move. But they probably wouldn't attack right away and play something else first.
The line before black could have played stronger in the end and win:
|1.g4 2.j15 3.q14 4.p11 5.l12 6.l10 7.j11 8.j9 9.h10 10.h8 11.f11 12.g6 13.o9 14.n11 15.p12 16.o13 17.q10 18.p18 19.t17 20.s21 21.q20 22.r19 23.n17 24.m17 25.l16 26.o18 27.o15 28.k16 29.f16 30.g20 31.i18 32.h14 33.e14 34.i19 35.j20 36.h17 37.h16 38.f13 39.d12 40.g15

no sorry that was a mistake, white has to play another 33rd move.
|1.g4 2.j15 3.q14 4.p11 5.l12 6.l10 7.j11 8.j9 9.h10 10.h8 11.f11 12.g6 13.o9 14.n11 15.p12 16.o13 17.q10 18.p18 19.t17 20.s21 21.q20 22.r19 23.n17 24.m17 25.l16 26.o18 27.o15 28.k16 29.f16 30.g20 31.i18 32.h14 33.g14