DM's Guild Author
The purple/deep dragon was exactly what I was thinking of when I wrote that. But, you'll still have the extra age categories for them at least. Also, I'm not sure how fast your turnaround would be, but you could fill any space from removing redundant dragons by adding in your extra age categories for other Fizban's dragons.
By the way, the multi-headed dragons look perfect to surprise players who venture into the lair of a dragon they've only heard rumors of, and suddenly find they will have to deal with a lot more than they bargained for!
It looks like WoTC changed the look of the deep dragon in Fizban's, which works out well for me. I can probably keep the purple dragon as is. I just need to change the lore a bit.