There are actually many versions of the firmware floating on the net right now with minor changes here and there. Some of them even overclocked the CPU to 1.1GHz.
The only ones I'm referring to are listed on https://github.com/retrogamehandheld/RG-350/wiki/Firmware-and-Software-Updates
. As to who made them, I don't know; I'll have to take your word on the fact that they're from different authors.
Do bear in mind that they're all "beta" releases, in that they may not be stable and still have noticeable bugs in them. For me, the best stable version is at 1.5.1, released by Tony Jih.
They're all unofficial in a sense as none of them are "official" OpenDingux releases. They're all based off of the last release of the GCW0 firmware.
I've also found that 1.5.1 is the "best" one for me. This CFW and the version of PSX4ALL seems to give me the best PSX performance. It won't play Bloody Roar II at full speed, but most other games I've tested look good.
The one and only reason I'm looking forward to a new firmware is for someone to add HDMI A/V out support. Sadly from what I've read here so far, no one's doing this feature at all or even be interested in doing it.
I was excited about the HDMI support, too. The idea that I might be able to attach an OTG USB-C hub for controllers, a separate one for charging, and an HDMI port would've made for a neat portable gaming device that could be hooked up to a television. From what I've managed to pick out from the forum, The GCW0 also had an HDMI port that was not implemented. There was *some* work in getting it to work, but doesn't seem that it ever worked well enough to put into firmware.
The RG-350 is really just a repackaged GCW0 (same CPU, RAM, similar screen, likely same GPU, no BT or wifi, though), so it's not surprising that something that didn't work on the GCW0 doesn't work on the RG-350. The developer is using what was a fairly open hardware design with an open operating system, so their development investment is very low. It's just the way a lot of these cheap emulation handheld companies operate. They put out a handheld and rely on the community to progress firmware development.