It's not really a problem for Bryston to solve as it relates to HDMI 2.1 since they don't build their own boards. In the case of the SP4, I believe that is MDS, I think that's who made the boards for the now essentially obsolete (not in the truest sense of the word but it's an outdated product none the less) Storm MK 1 platform which is what the SP4 is based on. With Storm having already moved to the MK 2 platform and also brining in a new partner for HDMI boards, moving away from MDS, with no future plan for the SP 4 announced by Bryston it's certainly looking like the SP4 would be stuck with what it has. I'm not sure if the old MK 1 platforms will be compatible with the new HDMI boards in the works at Storm. Also with Storm moving away from MDS it then may be up to Bryston to take the lead if they wish to continue to use MDS boards if they ever intend the SP4 platform to have HDMI 2.1.
The Sony A9G, that's a prior year OLED isn't it? If so it's 4k, not 8k and the gaming features on it are gimped to say the least with it not being HDMI 2.1 so for you at the moment HDMI 2.1 is irrelevant. Even the new Sony's this year are a joke for HDMI 2.1 implementation as they only have 2 inputs, one of which is eArc so any gamer having more than 1 gaming source has to manually unplug one gaming device and plug in another if they are taking advantage of eArch for any streaming services built into the tv. Also Sony has yet to implement VRR. But again, coming back to the Bryston situation, Sony, unlike LG and Samsung do not make their own HDMI boards and are at the mercy of the features built into the boards they do purchase, for Sony I believe it's mediatek and there is no guarantee Mediatek will ever be able to iron out the VR issues.