Welcome to the forum ! Personally I'm looking out for a 5800X3D to upgrade my AM4 build one last time, price/performance it's still hard to beat if you consider the amount of time and money needed to upgrade to a similar AM5 setup. If you need to change the motherboard anyway and don't have an mITX AM4 board yet, it might be worth figuring out wat both choices would cost you. As the performance with a 7800X3D is only going to be single digit percentages in general for 4K gaming but end up around 20% in 1080p gaming.
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Hope to see you around, enjoy the wonderful world of SFF !