Here's the fix!
How to upload to a YouTube channel, sub channel, managed channel, etc.
1. Switch to your personal account.
Click your avatar in the top-right, and click switch, then click on your account.
2. Click on the gear icon, and click on YouTube Settings and go to Advanced.
On the centre of the page, under Overview, Account Information, you'll find your avatar, and the word 'Advanced'.
3. Change your default channel to the one you want.
A new heading will have appeared, not visible when logged in as your channel, "Signing in to older apps." Underneath that, there is a drop-down menu that says Use My Current Account. Click it and select the channel you want to be the default upload channel.
You will still be able to choose channels on newer apps, but this will make the default channel whatever you choose on those apps which don't give you the option.
You're welcome!