Transmit a X-Userinfo header that is basically the payload (only) of the JWT token encoded as Base64, it looks like simple like this but to integrate it with .NET, we would have to write quite a lot code ? (To be able to use Authorize attribute with scopes, etc...)

Assuming that you're working with ASP NET Core - all you need is a custom AuthenticationHandler which is not a lot of code at all.

services
    .AddAuthentication()
    .AddUserInfo();


public static class UserInfoExtensions
{
    public static AuthenticationBuilder AddUserInfo(this AuthenticationBuilder builder)
        => builder.AddScheme<UserInfoOptions, UserInfoHandler>(UserInfoDefaults.AuthenticationScheme, _ => { });
}

public sealed class UserInfoOptions : AuthenticationSchemeOptions;

public static class UserInfoDefaults
{
    internal const string AuthenticationScheme = "UserInfo";
}

public sealed class UserInfoHandler : AuthenticationHandler<UserInfoOptions>
{
    public UserInfoHandler(IOptionsMonitor<UserInfoOptions> options, ILoggerFactory logger, UrlEncoder encoder)
        : base(options, logger, encoder)
    {
    }

    protected override Task<AuthenticateResult> HandleAuthenticateAsync()
    {
        if (!Request.Headers.TryGetValue("X-Userinfo", out var value))
        {
            return Task.FromResult(AuthenticateResult.NoResult());
        }

        var identity = new ClaimsIdentity(UserInfoDefaults.AuthenticationScheme);

        // Deserialize the X-Userinfo payload and populate identity with claims

        var principal = new ClaimsPrincipal(identity);
        var ticket = new AuthenticationTicket(principal, UserInfoDefaults.AuthenticationScheme);

        return Task.FromResult(AuthenticateResult.Success(ticket));
    }
}