That means that you can not compare those two devices in regards to OpenWrt support, and even if RT2600AC would be supported (which it is not), this would say nothing about support or supportability of RT1900AC because of different SoC in both devices.
About the chances to get this device supported:
- search the forum for the Broadcom SoC of RT1900AC -> zero results
- search the OpenWrt Table of Hardware for the SoC of this device: Out of 1430 devices there are zero devices listed with this SoC.
This tells you nothing about supportability, but that without any prior existing support for this SoC, you probably would need to start from scratch if you want to add OpenWrt support.
@slh You are usually better informed than me regarding linux support for SoC / Wifi, maybe you could give a short comment on this subject?