Emperor
Herald
Honestly, no point buying a new AM4 rig at that price point. For longevitiy, the B650 mobo you selected is good because of the 3rd M.2 slot & better VRM. I doubt gamers need more than 32GB RAM anytime soon, maybe another decade before we start hitting 32GB.
There is no major issue with Intel 12th Gen System.
I believe you can go with this instead of intel because new gen intel cpus are unstable and with ryzen you save some money for a better gpu instead of going am5 route. IMO the whole upgradable path selling point is bs to me because again I have to sell the old parts fro good price. Its better to buy the best value products and upgrade completely years after. For example the entry level am4 boards will not able to run the 5700x3d or 5900x today wtihtout thermal throttling. You pay a huge premium just for future proofing.
Your comparison of 5700x3d vs 12400 is not justified. Perhaps compare 5600 with 12400f for similar performance and then compare the price.