The Synology RT1900ac is a Linux based router with SSH, 256MB of DDR3, a dual-core 1GHz processor, and the stock OS has apps like VPN server, media server, QOS.
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?
There is no driver for BCM43460 (both wireless cards of the rt1900ac) at all, not even most basic support via b43.
The rt2600ac (ipq8065, very similar to the nbg6817) on the other hand should be easily supportable, if someone experienced/ motivated got their hands on one.
There was a thread on this forum from someone wanting to add support for it, but there was no interest from anyone so the effort might not have gone too far.