In the interest of keeping the comment small, I omitted a couple things:

    -p:Configuration=Release -p:DebuggerSupport=false 

FWIW, I did also try the following, but found they had basically no impact on output size:

    -p:TrimMode=link
    -p:EnableUnsafeBinaryFormatterSerialization=false 
    -p:EnableUnsafeUTF7Encoding=false 
    -p:EventSourceSupport=false 
    -p:HttpActivityPropagationSupport=false 

(Tested on .NET Core 3.1.5)

Huh, the debug symbols are in separate .pdb files, aren't they?

38MB is a pretty typical size for a real-world, self-contained, assembly-trimmed package, and the size comes from the runtime itself, plus the core assemblies that weren't trimmed out.

Actually, in my tests with the AoT compiler I had to strip the binary to get a significantly smaller one despite there being a .pdb file. Your mileage may vary as the build flags can be a bit fiddly.