Synthesizing the DNA itself is not the only problem however.
You then have to get that DNA into a working living form state so that normal cell processes can continue:
Apparently achieved for the first time in 2021: www.jcvi.org/research/first-self-replicating-synthetic-bacterial-cell by the J. Craig Venter Institute.
Basically a synonym for doing a large chunk of de novo DNA synthesis.
TODO: if we had cheap de novo DNA synthesis, how hard would it be to bootstrap a virus culture from that? github.com/cirosantilli/cirosantilli.github.io/issues/60
Is it easy to transfect a cell with the synthesized DNA, and get it to generate full infectious viral particles?
If so, then de novo DNA synthesis would be very similar to 3D printed guns: en.wikipedia.org/wiki/3D_printed_firearms.
It might already be possible to order dissimulated sequences online: