4-6 were made entirely separate from 1-3, 1-3 builds off of 4-6. But post-TRoS necessarily has to build off of TRoS, which really doesn’t lay any groundwork for anything worthwhile.
Eh, they could do what Star Trek did with The Next Generation: basically a “soft reboot” that doesn’t reboot the whole continuity, but focuses on new characters within the same universe. It’s not necessarily true that sequels have to depend fully on the movies that came before them, and I think Star Trek is one of the best examples of this. Other examples include book series of series’, like the Shadowhunters books by Cassandra Clair, which aren’t really soft reboots, but different series that occur in different time periods of the same universe and don’t always require you read one series to be able to read the other series. Comics also do this type of stuff, afaik.
Although, I do find it ironic that TFA was seen as an unwanted soft reboot, but a soft reboot of the sequel trilogy would probably be very much wanted, lol. Makes sense though.