Thanks for the advice, I figured out that the best way forward really was to use split_opacity but I thought that I could use f to work with alpha values separately because it would yield performance and RAM savings.
#@gui Layer cake : fx_layer_cake, fx_layer_cake_preview(1)
#@gui : note = note("Splits image into annular or circular layers and rotates each layer. Based on <a href="https://forums.getpaint.net/topic/26566-layer-cake-plugin/">the Paint.NET plugin</a>.")
#@gui : Iterations = int(4,1,32)
#@gui : Angle at centre = float(360,-1440,1440)
#@gui : Angle times iteration = bool(0)
#@gui : Size = float(75,0,200)
#@gui : Centre = point(50,50,0,1,255,255,255,175)
#@gui : Boundary = choice(3,"None","Nearest","Periodic","Mirror")
#@gui : Interpolation = choice(1,"None","Linear","Bicubic")
#@gui : Blur = float(0,0,200)
#@gui : sep = separator()
#@gui : Anti-alias amplitude = float(30,0,100)
#@gui : Edge threshold (%) = float(0,0,100)
#@gui : Smoothness = float(3,0,10)
#@gui : Output layers = choice("Off","Hollow","Filled")
#@gui : sep = separator(), Preview type = choice("Full","Forward horizontal","Forward vertical","Backward horizontal","Backward vertical","Duplicate top","Duplicate left","Duplicate bottom","Duplicate right","Duplicate horizontal","Duplicate vertical","Checkered","Checkered inverse"), Preview split = point(50,50,0,0,200,200,200,0,10,0)
fx_layer_cake :
repeat $! l[$>]
iter=$1
angle=$2
if $3 angle*=$iter fi
size=$4/2
to_rgba split_opacity to_rgb
repeat 2
part={1-$>}
l[$part]
repeat $iter
[0] rotate[-1] {$angle/$iter*($>+1)},$8,$7,$5%,$6%
100%,100%,1,1 ellipse. $5%,$6%,{$size/$iter*($iter-$>)}%,{$size/$iter*($iter-$>)}%,0,1,255
if {$13!=2} ellipse. $5%,$6%,{$size/$iter*($iter-1-$>)}%,{$size/$iter*($iter-1-$>)}%,0,1,1 fi
blur. {$9/$iter}
fx_smooth_antialias. ${10-12}
blend[-1,-2] multiply
done
if {!$13} blend[^0] add fi
if {$13!=2} 100%,100%,1,1 fc. 255,255,255 ellipse. $5%,$6%,{$size}%,{$size}%,0,1,0
blur. {$9/$iter}
fx_smooth_antialias. ${9-11}
blend[-1,0] multiply fi
if {!$13} blend add fi
if $part to_gray fi
endl
done
list={int($!/2)}
repeat {$list} a[$>,$list] c done
endl
done
fx_layer_cake_preview:
if {!$13} gui_split_preview "fx_layer_cake $*",${-3--1} else fx_layer_cake $* fi
Edit: added an interpolation option and a description of the command.