That person is a disaster for this community, also known as CornPOP and I've already seen several posts on reddit, where he desynces the game on purpose. Until recently, he had the Huskarl profile picture I think, seems like he's changing names a lot.
Apparently since a recent Steam update, you can't download depots with older manifests 24. But there is a workaround.
Check the latest comments in the link below:
I think you need to download from the depot "Phoenix Content", as that is, where the .exe file is located. The manifests for this depot are here: https://steamdb.info/depot/813781/manifests/
The files you downloaded are some game resources files, and they didn't change from the 21st January patch.
Yes, building your own PC is always the best option. Btw, I don't want to advertise anyone, but this build is pretty much what you're looking for. (You can maybe get a better GPU, if budget allows it.)
I think Ryzen 5 1600 AF revision and RX 570/GTX 1060 will be good enough for AoE4. Atm, R5 1600 AF revision is probably the best deal on the CPU market, cheaper than R5 2600, whilst having almost the same performance.