Correct, SunOS is a descendant of BSD and was the most widely used and renowned one during that time. And like you said, it was the gold standard for easy builds of most contemporary software and enthusiastic support.
DEC Ultrix is also BSD kin and IBM AOS and HP-BSD were intentionally vanilla BSDs. There were some commercial BSDs like Mt Xinu and BSDi that were episodically relevant.
BSD proper was alive and well especially in the academic and research circles into the 1990s and we get the current derivatives like Net, Free, and Open which are direct kin.
Mach is regularly BSD-affined because BSD was a typically ported server but Mach is decidedly its own thing (as a simple and drastic counterexample, there was MkLinux and OS/2 for PowerPC which had little to do with BSD but are very much Mach). NeXTSTEP and eventually Darwin/macOS inherit BSD affinity.
It had some heavy hitters behind it, but my understanding is DEC's effort was mired in organizational problems leading to fractured strategy and commitment. Many vendors were still figuring shared libraries out into the early 1990s so it must have been swept away from Ultrix once OSF/1 became the plan of record.
DEC Ultrix is also BSD kin and IBM AOS and HP-BSD were intentionally vanilla BSDs. There were some commercial BSDs like Mt Xinu and BSDi that were episodically relevant.
BSD proper was alive and well especially in the academic and research circles into the 1990s and we get the current derivatives like Net, Free, and Open which are direct kin.
Mach is regularly BSD-affined because BSD was a typically ported server but Mach is decidedly its own thing (as a simple and drastic counterexample, there was MkLinux and OS/2 for PowerPC which had little to do with BSD but are very much Mach). NeXTSTEP and eventually Darwin/macOS inherit BSD affinity.