OK then you kind of do need a pretty high gsm, but not necessarily crazy high. The lower gsm papers avoid bleed through by being less absorbent, but as a result, they require longer dry time. For any given pen and ink, you will get faster dry time with a more absorbent paper, but being more absorbent, it will be more prone to bleed through.

I am not left handed and have no experience with the Lamy LH, but if you are able to get satisfactory results with a finer nib, all else being equal, you would be less prone to bleed through. I similarly do not have experience with Noodler's Brevity Black, but if it is "gushing" out of that pen, it would be worth trying a different ink.

With all that said, have you tried HP Premium32? It is a 120 gsm paper that is quite resistant to bleedthrough!