If you’re doing YouTube videos, I’m not sure how much you’re really going to improve the sound with a cable. I’ve done this with an iPod Classic. There’s going to be a sweet spot of the volume on the iPad through the integrated. I’d find the right volume on the iPad where the signal is strong enough and use the integrated’s volume pot to turn it up and down.
There should be some docks or cables out there that bypass the ipad’s amplification. That should yield an improvement and not cost much.
I use an HRT iStreamer which takes the digital output and has its own DAC. Output to RCAs. It’s quite good. I don’t think they’re being made anymore, but they might be around on the used market. I can’t confirm it it works with your iPad, but it works with my iPhone SE. I’ll try my daughter’s iPad (the current plain iPad model) to see if that works too. I don’t see why it shouldn’t work because my iPhone is still a current model, but you know how that goes.