$30 is a small investment, slight though the improvement may be. Potentially that DDR4 will come in handy for a 12-14th gen build or an move to an AM4 system when the 5000 series really goes on sale in a year or two. It is possible DDR4 prices will start going up by then as they switch over to DDR5 production.
I agree, the 9900 is too expensive, as mentioned. Merely something I looked at.
i5-8400 tops out at 4Ghz, the i7-8700 boosts to 4.6Ghz and gets you hyperthreading. 600Mhz is nothing to sneeze at for gaming. If you can get one for $90 or less not a bad deal, and you can always sell the 8400 for a little bit.
If the PSU is running a 120W GPU reliably, I don't think an upgrade to the PSU is necessary for an RX6600. Any GPU larger and that is certainly a good idea.
Then we start getting into needing more detail. What games at what resolution and settings. For 1080p 60hz, not much reason to go further than the RX6600 at the moment. If the near future plan is a new 1440p monitor and a new CPU/Motherboard/RAM, then yes, investing in a larger GPU and PSU makes sense again. But if that is a more than a few years off, probably not worth doing.