Wireguard network with terrible bandwidth

Doubt it because the base station will be in Brazil as well, I guess.

it does not go through the satellites up to the other router?
I'm talking about two devices, one on the server and the other on the client.

Won't work. Starlink uses CGNAT. You can't run a server using Starlink. And no, it won't be satellite to satellite.

I installed the Shadowsocks on my Freenas that is on the server site.
And I installed the client on my Windows pc.
Using the AXT1800 to connect both sites and connecting to Shadowsocks with the local IP I got the same behavior, very small download compare to the upload.

Using also my local ISP modem and DDNS instead of local IP I got similar result.

This make sense?

This was the configuration I used on shadowsocks:
{
"server":"0.0.0.0",
"server_port":8388,
"local_port":1080,
"password":"*************",
"timeout":60,
"method":"aes-256-gcm"
}

And I think Shadowsocks uses TCP for the connection.

Isn't it just the same result? I mean ... like ... one ISP has problems?
I don't really see where this result is a proper test.

Also do you test it with wifi ?

Try restarting the wireless and also the wgclient interface.

I found a issue with a recent version of raw OpenWrt but it was too random to replicate it has some similarities on some op24 reports, my wifi also suddenly dropped to 50mb/s and 400mb/s upload, i believed mullvad had certain outtages aswell in the NL servers but the issue resolved when i restarted both.

Also it helped when i reconnected the device to wifi that did the final fix for me.

However in your case and what @admon said about the bad peering connection since there is not a good routing directly to EU, i think starlink also won't solve it, i guess starlink would just use the same route no?

Yes, it could conclude that ISP from Brazil have an issue maybe.
I will try this week to test with different ISP there.