[1.18.2] Better way to do my block-darkening fog of war feature?

I'm trying to do an RTS-style fog of war effect by darkening all blocks not within range of a particular mob.

My current method of doing this is to simply iterate over all blocks in the view cone (that are exposed to air) and draw a dark block around them using a vertexConsumer with this result:

my code: https://pastebin.com/SfmCGHKj

The problem with this is 3 fold:

• looks very glitchy with lots of tearing during camera movement
• doesn't account for non-solid blocks like tall grass - these show a visible black cube around them
• performance is terrible, my FPS drops by half whenever I run it

I can solve the weird grid-like effect on the water and the performance somewhat by drawing only block faces next to air but the other problems remain.

I can only think of two solutions to this either of which I have no idea how to implement:

1. GLSL shaders
2. Changing block light render values (clientside only)

Which way would be better, and/or is there another option I haven't thought of?

How difficult would either of these options be?