I’d ask the question as to whether optimising bin file extension sizes is all that important to begin with. Sure brotli at highest level compression has much smaller compressed file size versus gzip from my comparison benchmarks of various compression algorithms like gzip, xz, lzip, bzip2, brotli and multi threaded pigz, pxz, plzip, pbzip2, lbzip2 and zstd https://community.centminmod.com/threads/round-4-compression-comparison-benchmarks-zstd-vs-brotli-vs-pigz-vs-bzip2-vs-xz-etc.18669/.
But as these .bin files wouldn’t be natively loaded by a page’s HTML page, it isn’t going to affect page load speed as such.
And as you’re serving them from Cloudflare, it isn’t going to impact your bandwidth usage costs besides the initial non-cache hit/missed fetch from your origin. So only benefit is how long it takes a visitor to download the .bin files themselves. If you’re offering up .bin files for download, then using more common bzip2 or xz would be better in pre-compressed downloads i.e. .bin.xz or .bin.bzip2 files.
Not familiar with gltf formatted files but google search says they’re basically json files ?? So I guess highly compressible ? Tried using gzip compression using higher compression based Zopfli compression format ? pigz multi-threaded gzip can use Zopfli compression at level 11 specific compression level and the resulting .gz compressed file is easily decompressed with web browsers via gzip.