You might best use the way k1l0b1t is using it, this way you can do anything you want.
If you are keep using purely external DNS, you have to take in mind you can not use wildcards. That is not a problem I see.
However, you also have to manually copy over SPF an DKIM records if you use them, and I would advise to at least use SPF. Same for DMARC if used.
Also if you change A records, you have to manually put them in the external DNS also.
If that is not an issue, it's no problem to use purely external DNS.