Hi,
I can migrate your systems from NIS/yp to LDAP. It sounds like you want to run LDAP on your Windows Server(s). Could run LDAP on Linux or FreeBSD with kerberos, but if that's the case would have to coerce your Windows clients to join in.
I've been running Linux and Windows servers since the 1990's and FreeBSD servers for about 15 years.
I haven't dealt with Solaris for some years. Solaris 8 EOL was 2012. It would use LDAP on Windows 2000, but I've not tried a modern system. However, I found the install set (for Intel processors, should be same OS on Sparc) on Internet Archive web site and I will set up my own Solaris 8 virtual host to authenticate on my own Windows 2019 server so I have a proof and working setup.