Existing tool for converting gff3 to genbank (gbk)
I believe seqret is already the simplest approach. If you install the emboss command line utility, genome (file) sizes will not be a problem. You can even install it with conda, see https://anaconda.org/bioconda/emboss.

After that, you can use it as the example:

seqret -sequence {genome file} -feature -fformat gff -fopenfile {gff file} -osformat genbank -osname_outseq {output prefix} -ofdirectory_outseq gbk_file -auto


Hope it helps