I've never found any on the Amiga, but potentially something better than ARJm7 could be done.
Main idea could be to substitute huffman stage with tANS and do something for repeating LZ codes.
I've also somewhere a modified version of apLib based on exhaustive parsing (with no source available..) and a different bitcode (made by me), but packing ratio is very similar to apultra.
I too had brutally integrated ARJm7 into my loader, but in a completely impromptu and dedicated way, your code will probably be at least readable and commented, I'll check it.