I think your procedure is probably as quick as any.
The montage syntax for a custom layout is extremely tedious, it works with pairs of items so you have to build up from a base of two images. It ends up with a mountain of nested brackets. This for example, is only a 6 image, 2x3 stack:
V(V((H(0,1), H(2,3)),H(4,5)) Imagine a 12 x 8 combination.
One possibility is toggling visibility on selected layers, and using the input selected option. Fairly quick if you know how to toggle layer visibility and use ctl-f to repeat gmic.
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With a bit of editing you can end up with distributed layers, but a cumbersome process.
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Have you tried the alternative montage, Testing -> Afre -> Montage-X which has a slider to set the width