Best to import assets from CDNs or bundle them together with custom code?

So the 1st point here is why we use CDN:

A Content Delivery Network (CDN) works by providing alternative server nodes for users to download resources (usually static content like images and JavaScript). These nodes spread throughout the world, therefore being geographically closer to your users, ensuring a faster response and download time of content due to reduced latency.

so if you're planning to host your website across globe than I must say better you use CDN.

alternate thing is you can download and bundle using gulp as you are doing it right now but put your css file to some cdn. It will increase your performance. (and it's cheap)

here's amazon cloud front link :

https://aws.amazon.com/cloudfront/pricing/