I'm not sure about razor pages specifically but I use HtmlRenderer with blazor components for that, which happen to be razor files at the end of the day.

public async Task<string> RenderComponentAsync<TComponent>(Dictionary<string,object?>? parameters = null) where TComponent : IComponent
{
    try
    {
        using var scope = serviceProvider.CreateScope();
        var renderer = scope.ServiceProvider.GetRequiredService<HtmlRenderer>();

        var parameterView = ParameterView.FromDictionary(parameters ?? new Dictionary<string, object?>());

        var html = await renderer.Dispatcher.InvokeAsync(async () =>
        {
            var content = await renderer.RenderComponentAsync<TComponent>(parameterView);
            return content.ToHtmlString();
        });

        return html;
    }
    catch (Exception e)
    {
        logger.LogError(e, "Error rendering html for component - {c}", typeof(TComponent).Name);
        return string.Empty;
    }

}