As LLD originally said, the lack of updates is most likely linked to ASUS making the transition to 388. In addition, this wifi router looks to be primarily geared at serving as a mesh node. Mesh nodes don't run all the network control services, so ASUS may not be putting in as much effort to frequently update the firmware. Since you are using it as mesh nodes, and if your mesh is working well, I'm not sure I'd be concerned about a lack of recent updates, especially since some of the recent firmware updates have caused mesh node problems rather than improving things.
Your GT-AX11000 is the one that will benefit more from updates, and those have been coming out.