One of the poets who's vaguely (very vaguely) associated with the French symbolist movement, Guillaume Apollinaire, had a rather different approach to trench poetry. (He died at the end of the war, from a combination of wounds and influenza.) He, unlike many trench poets, was a professional writer very much at the forefront of technical experimentation. Like Mallarmé (that most symbolist of symbolists), he was fascinated by the materiality of language, by the question of what new horizons are opened to poetry by the fact that it's now something primarily encountered on the page. That's a change tacitly recognized by the development of free verse, in which line breaks become something seen more than heard. But he really pushes the limits of the "poetry as visual art" in his Calligrammes, in which he's playing with spatial relationships of words and the possibility of reading poems along multiple axes: usually there is one "direction" you can follow in order to get a fairly grammatically cohesive poem, but the mid-word line breaks or alignments of grammatically-separated words spatially side-by-side compels the reader to consider every element constituting the poem with renewed attention. It's certainly--though I've by no means devoted much attention to Apollinaire's work--another way of being "difficult" in the Modernist sense. The poems that directly refer to the war, such as the ones below (really, most of Calligrammes) also feature that secondary type of difficulty associated with the "derangement of the senses" (as Rimbaud would have put it) and disillusionment with a government that seems less sympathetic than the allemand in the opposing trenches.
Guillaume Apollinaire lived in Belgium for a while as a young man.
Long enough to master the Wallonian dialect and write a few poems in
it. Rather neat.