I'm not so sure an S-1 is necessarily all about or for an IPO only... S-1, as I understand is about registering shares... for example,
Progressive Care (RXMD) which utilizes our/
EagleForce's myVax is filing an S-1 to uplist to Nasdaq I believe... and I've seen an S-1 utilized for such before...
https://sec.report/Document/0001213900-21-052155/