Managed to get the monkey to load by patching simdjson 3.3.0 to fix a dubious 3 lines of code as mentioned here: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280590 then have fastgltf _NOT_ download simdjson again.

meson.build

@@ -81,7 +81,7 @@ sdl2 = subproject('sdl2').get_variable('sdl2_dep')
 
 fastgltf_opts = cmake.subproject_options()
 fastgltf_opts.add_cmake_defines({
-    'FASTGLTF_DOWNLOAD_SIMDJSON': false,
+  'FASTGLTF_DOWNLOAD_SIMDJSON': false,
 })
 
 fastgltf_proj = cmake.subproject('fastgltf', options: fastgltf_opts)
@@ -94,6 +94,7 @@ glm_opts.add_cmake_defines({
 glm_proj = cmake.subproject('glm', options: glm_opts)
 glm = glm_proj.get_variable('glm_dep')
 
+simdjson = dependency('simdjson')
 
 vk_opts = cmake.subproject_options()
 
@@ -120,6 +121,7 @@ dependencies += [
   imgui,
   sdl2,
   fastgltf,
+  simdjson,
 ]
 
 sources = [