I believe you had questions about gaming on that TV in a thread last year. The X90L supports HDMI 2.1 features and the full 48Gbps bandwidth of the Ultra Certification on HDMI ports #3 and #4 only as you already may know and the HDMI port #3 is also the eARC port. The X90L supports gaming in Dolby Vision @60Hz but not Dolby Vision @120Hz. It supports 1080p and 4K @120Hz but not 1440p @120Hz.
The PS5 does not support Dolby Vision at all and has an HDMI 2.1 port limited to 32Gbps. The XBOX Series X/S will be limited by the TV to gaming in Dolby Vision @60Hz. The XBOX Series X/S HDMI port supports HDMI 2.1 @40Gbps and HDR10 RGB 10 BIT Color @120Hz is supported and should be used in place of Dolby Vision @60Hz.
The Onkyo RZ70 supports 40Gbps and is equipped with a Web Setup feature that allows for very specific settings for each and every HDMI input port. You should be able to dial it all in and use only the Main HDMI output port. Make sure each HDMI port in the Onkyo is set to 8K Enhanced so that none of the HDMI 2.1 features are not crippled out of the gate.
If the RZ70 is connected to your network, type the web address into a web browser with the suffix: /websetup and enter ciuser as the User Name and Password. Once in, you will notice a list of settings on the left side of the screen. You want o go to the Limit Mode settings as these are only available for adjustment in the web setup.
The list of settings will vary by model. Here is a pic of the TX-NR6050 list.
Having said all of that there will be issues with the Main/Sub output feature in the RZ70 if HDMI-CEC is enabled in the TV(BraviaSync) and the RZ70(HDMI Control.) The TV will want to send an audio signal over HDMI port #3(eARC) when switching to HDMI port #4. Make sure HDMI-CEC is turned OFF in every single device. When using two output connections to the TV, make sure the Sub HDMI Output port is used, not the Zone 2 port, and make sure the Zone 2 HDMI setting is set to Not Use. Ultra Certified HDMI cables should be used all the way around in this system.
I’m not sure why the X90L has an issue with switching between VRR and Dolby Vision from the same HDMI port. I have an XBOX Series S and can output 4K Dolby Vision @120Hz through my Onkyo to my LG C1. All have 40Gbps HDMI 2.1 ports. I’ve made all of the necessary settings in the web setup feature for each HDMI port in the Onkyo as there is a mix of older and newer devices and some settings need to be adjusted for each device. Good luck with it all.
Onkyo RZ70 HDMI Signal Support Chart: