should I bother putting a DAC inbetween MacBook Pro and AudioEngine A5+??

The short answer I would suggest is yes. The DAC on the laptop is really not good. If you're looking for high quality audio production you'll need an external DAC. I would recommend the Schitt Modi 3 for $99. The audio quest dragonfly is an option too but I think you'll get more out of the schitt. 

A DAC would be an improvement. Schitt Modi is a great option. Need not spend more. Not sure about thunderbolt connection however. 

I guess it’d depend greatly on the quality of the Youtube recordings.  I’d be skeptical of that, but if they’re decent and you’re as sick as the rest of us here I’d definitely give it a try.  Given your computer is inherently a very noisy source, I’d even consider adding some kind of noise reduction device that have been shown to produce significant improvements.  I’d probably try a Topping E30 DAC and an iFi Silencer+ or iPurify3 to send it a cleaner signal to work with.  All told you’d be out $200 - $280, which in audiophile terms is next to nothing for a meaningful improvement.  I’m guessing you’d use an Apple Camera Adapter to get to USB out of your MacBook but not sure if that works with Thunderbolt (I use it from my iPhone)?

You might also also consider getting a Qobuz account (if you don’t already have one) that’d let you stream a whole world of CD-quality and hi-res music for $15/month and give that DAC something nice to work with.  Small price to pay for sanity on the road IMHO, or maybe just bill it to the client.  Anyway, hope this helps, and best of luck.